KB-1981 Send Email smart service fails to send emails for some processes with a 'sender thread limit exceeded' error

Symptoms

When sending emails using the Send Email Smart Service, some emails fail with the following error present in the application server log:

ERROR com.appiancorp.ap2.mail.Mail - Could not send mail via <SMTP_SERVER>:<smtp_port> to
ERROR com.appiancorp.process.runtime.activities.SendEmailActivity - com.sun.mail.smtp.SMTPSendFailedException: 432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded [Hostname=<HOSTNAME>]
com.sun.mail.smtp.SMTPSendFailedException: 432 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded [Hostname=<HOSTNAME>]

Cause

This error appears when using Microsoft Exchange Online. As of June 1, 2018, Microsoft's SMTP Authenticated Submission protocol has been changed in order to only allow three concurrent connections to the service per mailbox. Additional connections are rejected with the above error. The complete documentation regarding this limit can be found here.

Action

To avoid this error:

Use another mail server that can handle more concurrent connections.
If the issue is happening on an Appian Cloud environment, use the Appian Cloud mail server instead of an external mail server.

Affected Versions

This article applies to all versions of Appian.

Last Reviewed: August 2019