CiviCRM Community Forums (archive)

I submit a mass mail job, about 1400 recipients.
Some addresses (about 400) give bounces with Syntax error code.
If I submit a new job for those bounced recipients, the job works fine and all messages are sent. And in effect the mail addresses are correct and verified.
Why?
Thank you.

For anyone else stumbling here, a possible reason is mail sending throttling. For example, one of our hosts has a hard limit of 625 mails/hour and discards all other mails. It produces a bounce for each, but its message is currently not recognized, so it ends up in the Syntax category.