Replies from zimbra being bounced by secureserver.net

[stolen time]

I am using Zimbra 8.8.15_GA3928 on Ubuntu 18.04. It (and earlier versions) has been running well enough with my current hosting service provider, for just under two years. In the last month they have started routing the mail via a new anti-spam system, secureserver.net, and it has started to bounce mails which clearly are not spam. I hypothesise it's because Zimbra is inserting headers into the mail which confuse secureserver.net. It seems to be only a subset of reply messages that get bounced, and all messages created with "new message" get through. A typical bounce-back error message is:

host wbeout.secureserver.net[173.201.193.160] said: 552
User has exceeded its 24-hour sending limit. Messages to 6868 recipients
out of 1500 allowed have been sent. (in reply to end of DATA command)

This isn't a terribly clear message, it looks very much is if there is meant to be a username after "User". My service provider has agreed that it's not a problem with my Zimbra installation sending too many messages, but won't investigate further because they can't reproduce the problem using their own system. I have changed all the passwords of course but it anyway doesn't seem likely to be a result of someone else impersonating me to send spam, as then all e-mail would presumably be blocked not just replies.

Has anyone else had this problem and found a work-around, by suppressing SMTP headers or similar?

[DualBoot]

Hello,

it seems your sender has been hacked or something else has gone wrong with his account.

Regards,