Why is Microsoft Office 365 blocking my account (flagging as a spam account)?

Now and again some Microsoft 365 security settings or automatic filters are flagging mailboxes that use MailMerge365 with "spam behaviour". The plugin itself doesn't act any other way that your standard mail merge with Excel and Word and we go the extra mile to stay compliant (e.g. only allowing 50% of the daily allowance of mailbox sending) but the fact of the matter is these filters are still triggered sometimes. The standard use cases we hear when this happens are:

• You setup a new mailbox and started sending large amounts of mails suddenly. 
  • In this case: slowly ramp up your use, this is generally good advice as the receiving end will also flag a sudden amount of email from a specific sender as suspicious.
• You are changing mailbox settings regularly to send campaigns with the same mailbox but with different "From" names.
  • In this case: use one mailbox for one "From" address
• The contents of your email share properties with spam emails (e.g. lot's of HTML, colors, pictures, links).
  • In this case: try sending at test email to the email provided at https://www.mail-tester.com/ to see what a common spam filter would say about your contents and remove some of the more "spammy" looking items. 

What else can you do about it?

In the end if the above things don't help the only way around this is to whitelist (allow list) the mailbox in question as to show Microsoft 365 that the sending is intended and not malicious. You will need to contact your IT administration to get this setup.