Vert.x Mail client (SMTP client implementation)

Vert.x client for sending SMTP emails via a local mail server (e.g. postfix) or by external mail server (e.g. googlemail or aol).

The client supports a few additional auth methods like DIGEST-MD5 and has full support for TLS and SSL and is completely asynchronous. The client supports connection pooling to keep connections open to be reused.

To use this project, add the following dependency to the dependencies section of your build descriptor:

  • Maven (in your pom.xml):

  • Gradle (in your build.gradle file):

compile 'io.vertx:vertx-mail-client:4.1.8'

Creating a client

You can send mails by creating a client that opens SMTP connections from the local jvm.

The client uses a configuration object, the default config is created as empty object and will connect to localhost port 25, which should be ok in a standard Linux environment where you have Postfix or similar mail server running on the local machine. For all possible properties of the config object, see below.

The client can use a connection pool of the SMTP connections to get rid of the overhead of connecting each time to the server, negotiating TLS and login (this function can be turned off by setting keepAlive = false). A client can either be shared or non-shared, if it is shared, the same connection pool will be used for all clients using the same identifier.

MailConfig config = new MailConfig();
MailClient mailClient = MailClient
  .createShared(vertx, config, "exampleclient");

The first call to MailClient.createShared will actually create the pool with the specified config. Subsequent calls will return a new client instance that uses the same pool, so the configuration won’t be used.

If you leave out the pool identifier, a default pool will be created. Note that the clients are shared in the scope of a vertx instance only (so two different vertx will have different pools with the same identifier).

The unshared client can be created the same way leaving out the identifier.

MailConfig config = new MailConfig();
MailClient mailClient = MailClient.create(vertx, config);

A more elaborate example using a mailserver that requires login via TLS

MailConfig config = new MailConfig();
MailClient mailClient = MailClient.create(vertx, config);

Sending mails

Once the client object is created, you can use it to send mails. Since the sending of the mails works asynchronous in vert.x, the result handler will be called when the mail operation finishes. You can start many mail send operations in parallel, the connection pool will limit the number of concurrent operations so that new operations will wait in queue if no slots are available.

A mail message is constructed as JSON. The MailMessage object has properties from, to, cc, bcc, subject, text, html etc. Depending on which values are set, the format of the generated MIME message will vary. The recipient address properties can either be a single address or a list of addresses.

The MIME encoder supports us-ascii (7bit) headers/messages and utf8 (usually quoted-printable) headers/messages

MailMessage message = new MailMessage();
message.setFrom("[email protected] (Example User)");
message.setTo("[email protected]");
message.setCc("Another User <[email protected]>");
message.setText("this is the plain message text");
message.setHtml("this is html text <a href=\"\"></a>");

Attachments can be created by the MailAttachment object using data stored in a Buffer, this supports base64 attachments.

MailAttachment attachment = MailAttachment.create();
attachment.setData(Buffer.buffer("attachment file"));


When using inline attachments (usually images), it is possible to reference the images within a html message to display html with the images included in the mail. Images can be referenced as <img src="cid:contentid@domain"> in the html text, the corresponding image has Disposition: inline and the Content-ID header as "<contentid@domain>". Please note that RFC 2392 requires Content-ID values to be structured like a Message-ID with angle brackets and a local and domain part using URL compatible encoding. None of this is not enforced and most mail clients supports IDs without angle brackets or without domain part, the best practice is to use the strict format. A valid example for a Content-ID value is "<filename%[email protected]>"

MailAttachment attachment = MailAttachment.create();
attachment.setData(Buffer.buffer("image data"));
attachment.setContentId("<[email protected]>");


When sending the mail, you can provide a AsyncResult<MailResult> handler that will be called when the send operation is finished or it failed.

A mail is sent as follows:


DKIM Signature Signing emails

It supports DomainKeys Identified Mail (DKIM) Signature signing to secure your emails. All you need to do is to specify required configurations to sign your email.

A mail client with DKIM feature enabled can be created as follows:

DKIMSignOptions dkimSignOptions = new DKIMSignOptions();
dkimSignOptions.setPrivateKey("PKCS8 Private Key Base64 String");
dkimSignOptions.setAuid("[email protected]");
MailConfig config = new MailConfig()

MailClient mailClient = MailClient.createShared(vertx, config);

After the mail client is created, each mailClient.sendMail call will have the email signed by adding additional DKIM-Signature header.

Caching the Attachment Streams used in DKIM

To be able to perform DKIM sign, it needs to hash the email’s body, including attachments. If an attachment comes from a ReadStream, it won’t be able to go through again. So we need to cache the attachment data. This client provides 2 strategies to cache it.

  • In memory caching

By default, the stream content is cached in memory so it can be sent later.

  • Caching in a temporary file

You can cache data from attachment’s Stream to a temporary file by specifying a system property: vertx.mail.attachment.cache.file to true for large attachments. It will try to delete the temporary file after each send.

Mail-client data objects

MailMessage properties

Email fields are Strings using the common formats for email with or without real name

The MailMessage object has the following properties

  • from String representing the From address and the MAIL FROM field

  • to String or list of String representing the To addresses and the RCPT TO fields

  • cc same as to

  • bcc same as to

  • bounceAddress String representing the error address (MAIL FROM), if not set from is used

  • text String representing the text/plain part of the mail

  • html String representing the text/html part of the mail

  • attachment MailAttachment or list of MailAttachment attachments of the message

  • inlineAttachment MailAttachment or list of MailAttachment of inline attachments of the message (usually images)

  • headers MultiMap representing headers to be added in addition to the headers necessary for the MIME Message

  • fixedHeaders boolean if true, only the headers provided as headers property will be set in the generated message

the last two properties allow manipulating the generate messages with custom headers, e.g. providing a message-id chosen by the calling program or setting different headers than would be generated by default. Unless you know what you are doing, this may generate invalid messages.

MailAttachment properties

The MailAttachment object has the following properties

  • data Buffer containing the binary data of the attachment

  • stream ReadStream that represents the source of the binary data of the attachment

  • size int describing the attachment size when using stream as the source of the binary data

  • contentType String of the Content-Type of the attachment (e.g. text/plain or text/plain; charset="UTF8", default is application/octet-stream)

  • description String describing the attachment (this is put in the description header of the attachment), optional

  • disposition String describing the disposition of the attachment (this is either "inline" or "attachment", default is attachment)

  • name String filename of the attachment (this is put into the disposition and in the Content-Type headers of the attachment), optional

  • contentId String describing the Content-Id of the attachment (this is used to identify inline images), optional

  • headers MultiMap of headers for the attachment in addition to the default ones, optional

MailConfig options

The configuration has the following properties

  • hostname the hostname of the smtp server to connect to (default is localhost)

  • port the port of the smtp server to connect to (default is 25)

  • startTLS StartTLSOptions either DISABLED, OPTIONAL or REQUIRED, default is OPTIONAL

  • login LoginOption either DISABLED, NONE or REQUIRED, default is NONE

  • username String of the username to be used for login (required only when LoginOption is REQUIRED)

  • password String of the password to be used for login (required only when LoginOption is REQUIRED)

  • ssl boolean whether to use ssl on connect to the mail server (default is false), set this to use a port 465 ssl connection (default is false)

  • ehloHostname String to used in EHLO and for creating the message-id, if not set, the own hostname will be used, which may not be a good choice if it doesn’t contain a FQDN or is localhost (optional)

  • authMethods String space separated list of allowed auth methods, this can be used to disallow some auth methods or define one required auth method (optional)

  • keepAlive boolean if connection pooling is enabled (default is true)

  • maxPoolSize int max number of open connections kept in the pool or to be opened at one time (regardless if pooling is enabled or not), default is 10

  • trustAll boolean whether to accept all certs from the server (default is false)

  • keyStore String the key store filename, this can be used to trust a server cert that is custom generated (optional)

  • keyStorePassword String password used to decrypt the key store (optional)

  • allowRcptErrors boolean if true, sending continues if a recipient address is not accepted and the mail will be sent if at least one address is accepted (default false)

  • disableEsmtp boolean if true, ESMTP-related commands will not be used (set if your smtp server doesn’t even give a proper error response code for the EHLO command) (default false)

  • userAgent String represents the Mail User Agent(MUA) name used to generate email boundaries for multipart emails and message-id, default is vertxmail.

  • enableDKIM boolean if true, the DKIM signing will be enabled if DKIM configurations are set as well, default is false.

  • dkimSignOptions List of DKIMSignOptions which are used to perform the DKIM sign.

  • pipelining enables pipelining if the SMTP server supports it. Default is true

  • multiPartOnly boolean, if encode mail messages as multipart only or not. Default is false

  • poolCleanerPeriod int, the connection pool cleaner period in milli seconds. Default is 1000 ms.

  • poolCleanerPeriodUnit TimeUnit of cleaning the pool, Default is TimeUnit.MILLISECONDS

  • keepAliveTimeout int, the keep alive timeout for SMTP connection in seconds. Default is 300 s.

  • keepAliveTimeoutUnit, TimeUnit of keeping connections in the pool alive. Default is TimeUnit.SECONDS

  • ntDomain, String, the domain name used for NTLM authentication. If username follows format: <DOMAIN>\<UserName>, the part before \ will be used for the domain name.

  • workstation, String, the workstation name used for NTLM authentication

MailResult object

The MailResult object has the following members

  • messageID the Message-ID of the generated mail

  • recipients the list of recipients the mail was sent to (if allowRcptErrors is true, this may be fewer than the intended recipients)

DKIMSignOptions object

The DKIMSignOptions object has the following properties

  • privateKey The RSA PKCS#8 format private key used to sign the emails.

  • privateKeyPath The file path where the RSA PKCS#8 format private key is specified. Either privateKey or privateKeyPath is required.

  • signAlgo either DKIMSignAlgorithm.RSA_SHA256(default) or DKIMSignAlgorithm.RSA_SHA1. The algorithm used to do the body hashing and signature sign.

  • signedHeaders List of String that specify which email headers will be used to perform the sign. Defaults: From, Reply-to, Subject, Date, To, Cc. Note: the order matters.

  • sdid required, String, Singing Domain Identifier(SDID), normally it is the domain of the SMTP server.

  • auid optional, String, the Agent or User Identifier(AUID), default is @ plus sdid

  • selector required, String, the selector used to query public key.

  • headerCanonAlgo Canonicalization algorithm used for mail headers, one of simple(default) and relaxed.

  • bodyCanonAlgo Canonicalization algorithm used for mail body, one of simple(default) and relaxed.

  • bodyLimit optional, int, how long of the body used to calculate the body hash.

  • signatureTimestamp optional, boolean, if includes timestamp in the DKIM-SIgnature tags list. default is false

  • expireTime optional, long, expire time in seconds when the signature sign will be expired from now.

  • copiedHeaders optional, List of strings, the copied headers used in DKIM. Usually they are used for debug purpose according to the DKIM spec.