How do I add a Domain to Microsoft Whitelist

You can try to and get off Microsoft's blacklist by filling out this form (https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_1.0.0.0&wfname=capsub&productkey=edfsmsbl3)

When submitting if you get an error that means there's too much info in one of the text fields. Correct that and then submit.

You can try to and get off Microsoft's blacklist by filling out this form (https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_1.0.0.0&wfname=capsub&productkey=edfsmsbl3)

When submitting if you get an error that means there's too much info in one of the text fields. Correct that and then submit.

Then wait for an answer.

I've tried that. Unfortunately, they tell me they only investigate i.p's and not domains or accounts

If that's the case then it is likely the email you send triggers the spam detectors. Unfortunately I don't know what you'd have to do to change your email that you send so that it doesn't trigger those detectors.

Good luck!

OK, thank you for your suggestions.

Just to add another curve. the domain is relatively new and the only email which goes to spam is "paul@.....", which is my personal and main email. A new account I created "info@....." and "accounts@...." work fine !!! 

Then it is likely some people who have received emails from "paul@" have classified them as spam so now Microsoft's system just automatically classifies them as such.

With time, if enough people classify your new emails as spam, then they too will be classified as spam too.