Email to self Undeliverable

If an email to myself using eM Client (or have myself in the list of recipients),
I get a message back from Microsoft Outlook “Undeliverable” with "Remote Server returned ‘550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)’ " in the detail.

If I bypass eM Client, and send the same email direct using Microsoft Outlook, eM Client picks up the mail ok. Similarly if I send from gmail, eM client works ok.

So it looks like a problem on sending from eM Client - I am sure this used to work ok.

Problem is limited to eM Client -> MS Outlook. Have tried the same test with my Gmail account, and that works.
Have also tried deleting the Outlook account and setting it up again, but the problem comes back.

Has started working now, mail addressed to myself is being delivered.
I dont think its because of anything I did and suspect MS Outlook have made a change.