I started getting this on our transactional mail relay some years ago, hotmail responded with "Not qualified for mitigation", therefore instead of spending poin

I started getting this on our transactional mail relay some years ago, hotmail responded with "Not qualified for mitigation", therefore instead of spending pointless effort dealing with these miserable jollems i used o365 to counter it (my organization was already using o365, which also never did this S3150 thing to me) by: Adding a relay user to the o365 then configuring all the domains i needed to send from on the o365 tenant (without changing the MX records naturally). (Using a normal o365 user account to relay is possible for me as our typical customer base is enterprise and education, not random end users) Adding distribution lists as the from-addresses that i needed to send from and gave send-as rights to the relay user, compiled a list of hotmail/outlook domains (which i've had to add to over the years but it is relatively stable for me by now), then configured the outgoing relay server that does all the dkim-stamping to route all outgoing mail to outlook/hotmail domains through o365. When i see a S3150 in the logs i just add another domain to the list, i should probably make this pre-emptive by looking at the target mx records before sending, as its not trivial to make exim resend something that ended in a 5xx. All other mail providers have removed blocks by simply asking, not hotmail. Also all our newsletter-like mail goes through separate services, these are all transactional mails (meeting invites, user account creation, password resets and such).