550 5.7.350 Remote server returned message detected as spam - mails with zipped attachments

[AdrianD_RWA]

Hi Exchange Community,

I am having many emails failing to deliver to recipients on Exchange Online.

This has only started in the past 10 days. All the emails have zipped files attached (most under 1Mb). However, they are failing with the error 550 5.7.350. 

The recipients are receiving all other emails from us, so I don't believe it's a whitelisting issue. Anybody any ideas what could be the issue?

Many thanks in advance

Adrian

[#2]

Best check with the admins on their end, if possible.