God, yes. Gmail, in particular, really enjoyed just dropping connections from my IPv6 addresses every few months. Sometimes it would return an error, something obtuse and wrong like maybe, "your forward and reverse DNS records do not match" even though they absolutely did and hadn't changed in years.
Outlook/Hotmail was far better and would always return a sane, yet sometimes inscrutable, error that bore some resemblance to reality. It still took them a week to get me off the hit list that I got dropped on for no discernible reason.
> Or they decide to mark your mail is "suspicious" and it goes into the recipient's Spam folder.
Gmail was the ABSOLUTE FUCKING WORST about this. My spouse's e-mail to Etsy customers who used Gmail--not spam, not newsletters, person-to-person e-mails INITIATED BY THE CUSTOMER--would work for the first couple of e-mails in a thread and then start going into spam. Or, even more fun, friends with whom either of us had e-mailed would send us a mail and then we'd reply and nope, into spam.
Gmail would do this for WEEKS and no amount of poking or begging or pleading would make it stop. And then it spontaneously would. So maybe the begging did work but was just delayed. I dunno. I tried signing up for Gmail's "Postmaster Tools" but no data was ever reported because "your mail volume is too low."
The ultimate ignominy? The final humiliation that sent me to Fastmail? I finally, through a couple of professional contacts, got to actually e-mail (via Gmail, of course, because they were blocking my personal domain again) with a real, live Gmail SMTP admin to ask why my latest round of e-mails were going into the bit bucket. Know what he said, even though my domain has been hosted on a server in the same colo facility with the same IP addresses for a decade? Something along the lines of "your domain reputation is coming back as too new."
Ladies and gentlemen, I give you whois:
google.com - Creation Date: 1997-09-15T04:00:00Z
mypersonaldomain-notreallythisvalue.org - Creation Date: 1997-03-10T05:00:00Z
