Check Blacklists - scottgoetz/ansible GitHub Wiki
https://mxtoolbox.com/blacklists.aspx
https://ipcheck.proofpoint.com/
Special Note
Microsoft has a blacklist that is unpublished and non-searchable that takes precedent over a clients/targets whitelist. This special blacklist will block your emails even if your client whitelisted your infrastructure and your servers show up clean on every blacklist, including https://sender.office.com/.
You'll know if you're on this special blacklist if you send an email to an Outlook inbox and get Undelivered Mail return messages for each email sent from the targets mail server that looks like the image below:
To fix this issue, follow the instructions in the Undelivered Mail return email! Send an email to [email protected] requesting to be taken off of this blacklist.
There is a trick to this though, Microsoft will reply back and ask you to submit your server to https://sender.office.com/ then respond after a period of time. This is where it gets tricky.. You have to respond via a Forwarded message to [email protected] and not a reply to the person handling your ticket because that email address is behind this dumb blacklist!