Bulk email failure via Sophos XG

[Quatermass]

We use EduLink, Sophos XG and on-site Exchange. We've found that a "mass email" from EduLink to parents (around 1500 recipients) is failing after 300 recipients (probably 3 batches of 100, i.e. 3 tries from EduLink hitting a 100 limit each time, then EduLink gives up). XG is acting as an MTA so the emails don't actually touch our Exchange (though we've set the recipient limits high on there anyway just to be sure).

 

So it looks like an XG setting is the issue (EduLink email failure log shows SMTP 421 of "too many messages in this connection"). The only thing I can find on XG is Email > General Settings > SMTP Settings > SMTP DoS settings - which can indeed be set to 100. BUT these SMTP DoS settings are NOT enabled here!

 

Thoughts?

Edited January 22, 2020 by Quatermass