> Also, most of the issues will not be with obscure email servers, they will be with Outlook and Gmail. When you send email, you will never know if your email will be delivered to inbox, delivered to spam folder, or blackholed entirely.

In my personal experience, over five years of running my own business and personal email entirely and another decade and counting since of running PBXes that directly send voicemail messages, Gmail has only explicitly blocked me once, and that was 100% legitimate because the site the PBX was located at had an infected PC. It occasionally will filter messages to the spam folder, but that almost always means one of the users actually flagged a message.

I could send an email to myself from a bogus address via raw SMTP over Telnet to port 25 and I'd be willing to bet my week's pay that Gmail delivers it. They might mark it as spam, but they deliver reliably.

Microsoft on the other hand is a massive pain in my ass. They usually work, but about 4-5 times a year they seem to randomly pick one of my systems to treat as suspicious and block emails from. The process listed in their block message always works, but it's annoying to have to deal with regularly, especially with no rhyme or reason.

Yahoo is the worst, they straight up block everything I do with no recourse. I have one user who insists on using his Yahoo email and I basically had to tell him we don't care that Yahoo won't accept messages.