To share one trick I found (after you've done all the DKIM/SPF/DMARC/PTR things others point out), if you still have deliverability issues to Microsoft 365 tena

To share one trick I found (after you've done all the DKIM/SPF/DMARC/PTR things others point out), if you still have deliverability issues to Microsoft 365 tenants, it is worth sending mail as HTML rather than plain text. I have no idea why that helps (well, I could guess that some spam heuristic thinks plaintext email without an accompanying HTML envelope is more likely to be spam), but changing this took me from near-constant "your email went to spam" to no issues sending even things that actually look spammy (i.e. an email just containing a link that might be of interest to the recipient)