I would consider doing it for connection refused or failed connections, but I would only retry a couple of times so it didn't take too long.

Maybe you can consider a queueing mechanism (asynchronously), but a few retries would already be great!

I don't think a queuing mechanism would be a good idea. I'll look into retries for specific issues (socket exceptions, etc).

IMHO Jenkins is not responsible for reliable e-mail delivery (if you want such thing, you should make your e-mail infrastructure reliable). But some - not too many! - retries probably won't hurt!
But remember that this may needlessly delay the completion of jobs if the e-mail server continues to be unavailable.

I agree, this is why I will restrict the number of retries to maybe even just one.

One retry would already be nice!
Any idea when this can be implemented?

Code changed in jenkins
User: Alex Earl
Path:
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java
src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherDescriptorTest.java
http://jenkins-ci.org/commit/email-ext-plugin/3af25cc2606cf0147376870d32de5930521e6a6f
Log:
Fix JENKINS-16181

• Added a retry on SocketException occuring
• Added commented out main descriptor test, this will be used later

Compare: https://github.com/jenkinsci/email-ext-plugin/compare/52dae3a93c5a...3af25cc2606c

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Added a retry on SocketException occuring.

I'm using 2.27.1 and i'm not seeing any retry messages.
Email was triggered for: Success
Sending email for trigger: Success
Sending email to: xxx@xx.com
ERROR: Could not send email as a part of the post-build publishers.
javax.mail.MessagingException: Could not connect to SMTP host: smtp.xx.com, port: 25;
nested exception is:
java.net.ConnectException: Connection refused
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1282)
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:370)
at javax.mail.Service.connect(Service.java:275)
at javax.mail.Service.connect(Service.java:156)
at javax.mail.Service.connect(Service.java:105)
at javax.mail.Transport.send0(Transport.java:168)
at javax.mail.Transport.send(Transport.java:98)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:317)
at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:296)
at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:256)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:703)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:678)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:656)
at hudson.model.Build$RunnerImpl.cleanUp(Build.java:171)
at hudson.model.Run.run(Run.java:1452)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:337)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:198)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:180)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:391)
at java.net.Socket.connect(Socket.java:579)
at com.sun.mail.util.SocketFetcher.createSocket(SocketFetcher.java:230)
at com.sun.mail.util.SocketFetcher.getSocket(SocketFetcher.java:189)
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1250)
... 18 more

Retry will currently only occur on SocketExceptions, would need to add something for ConnectException

Adding ConnectException would be very helpful. Do you think it is possible?

Will add for the next release

Apparently, retry handling for the "connetion refused" case does not yet work properly.

I do observe the following stack trace whenever the SMTP server is unreachable:

javax.mail.MessagingException: Could not connect to SMTP host: mail.example.net, port: 25;
nested exception is:
java.net.ConnectException: Connection refused
at com.sun.mail.smtp.SMTPTransport.openServer(SMTPTransport.java:1934)
at com.sun.mail.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:638)
at javax.mail.Service.connect(Service.java:295)
at javax.mail.Service.connect(Service.java:176)
at javax.mail.Service.connect(Service.java:125)
at javax.mail.Transport.send0(Transport.java:194)
at javax.mail.Transport.send(Transport.java:124)
at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:305)

...but I can never see a "Socket error sending email, retrying once more in 10 seconds" message.

I suspect that the root cause is that Transport.send() will not throw a SendFailedException in this case, but rather a MessagingException. However, the plugin code only catches SendFailedException.

(Tested with version email-ext-2.37.2.2, but exception type handling is identical on master.)

Could you please check?

Alexander, I am seeing the exact same issue as you and it started at around the same time you posted your comment. I am only seeing it intermittantly though, is it the same for you or are you seeing it with every email? Perhaps this is a new issue?
I looked for an option to specify the number of retries for an email failure but couldn't find one. Does a feature like this exist?

There is no option to specify the number of retries.

Added handling when a MessagingException is caused by ConnectException. Identical retry handling as for SendFailedException.
PR available at https://github.com/jenkinsci/email-ext-plugin/pull/96

Code changed in jenkins
User: Christian Galsterer
Path:
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java
http://jenkins-ci.org/commit/email-ext-plugin/531a9232658a3bb725be1f0551a1bb01ec30c588
Log:
JENKINS-16181 Jenkins does not try to resend a failed e-mail

• Added handling where MessagingConnection is caused by a ConnectException. One retry with a delay of 10 sec is made.

Code changed in jenkins
User: Alex Earl
Path:
src/main/java/hudson/plugins/emailext/ExtendedEmailPublisher.java
http://jenkins-ci.org/commit/email-ext-plugin/99ff8aed9777edfd24420ed3ba91a6d9f322968a
Log:
Merge pull request #96 from christiangalsterer/JENKINS-16181

JENKINS-16181 Jenkins does not try to resend a failed e-mail

Compare: https://github.com/jenkinsci/email-ext-plugin/compare/5a0f29b55751...99ff8aed9777

Fixed in 2.39