Office 365 at least has several well known bugs:<p>1. Silently dropping email (no smtp transaction bounce, not in junk, no rules in place) for a specific single sender at a domain, where email from other senders at the same domain is accepted. Hundreds of people have posted about this online with no fix or acknowledgement from Microsoft.<p>2. Overly strict hardfail SPF policy by default for new customers who have no idea of SPF and should be using softfail. These customers often use third party systems to send out hundreds of account statements. These get blocked by other email hosts. The customers themselves have no idea and never correct their SPF, saying something like &quot;it must be an issue on your side&quot;.<p>3. Blocking entire &#x2F;24 IP ranges including good senders with good reputation, on the basis of only a single spammer in the range. This particularly affects those in datacenters, even if you are on a dedicated machine and your IP is good.<p>4. It may be that Office 365 does not track or measure domain reputation at all, which makes things like SPF or DKIM of little positive use, and introduces loads of spam false positive bounces.