Symptom

Notification emails fail with a "Too many recipients" exception, observed in the outgoing-mail.log:

com.atlassian.mail.MailException: com.sun.mail.smtp.SMTPSendFailedException: 452 4.3.1 Insufficient system resources

Causes

This error code is coming from the MS Exchange SMTP server rather than Jira or Java Mail. This indicates a lack of resources on the MS exchange mail server. Check that there is sufficient disk space and memory on the MS Exchange server. 

Diagnosis

Enable mail protocol logging to determine if the 452 error codes are appearing in the log entry.

Resolution

Free up disk space or allocate additional memory in the MS Exchange Server that Jira is connecting to.