Wrong bounces

[vasacampana]

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.

[adixon]

Those numbers suggest that something happens after the first 1000. Try looking for the pattern of which ones are bouncing.

[lynx]

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.