In an effort to reduce spam and fraud, email hosts are locking down security making the way we used to use email difficult. Even after setting up your domain perfectly with an SPF Record (Sender Policy Frame), a DKIM record (Domainkeys Identified Mail), and a DMARC record (Domain-based Message Authentication, Reporting, and Conformance); if you send out tens of thousands of emails a day, you will still be flagged by many carriers as an offender, even if the emails are all legitimate. Email now requires monitoring, especially for large organizations. Lack of monitoring lead to the block placed by AT&T on Microsoft. Microsoft "trusted" its users to perform their own in house security and monitoring which lead to widespread abuse and negligence causing rampant spam from their servers which now affects those who did the right thing as well as those that did not. The next time an email "didn't go through", ask them when their last domain review was and refer them to Jonathan Ongtingco.