Some info for anyone who might come across the same problem I did today:
This morning I woke up, expecting the usual SnapRAID daily email from my server, but was surprised to see nothing in my Inbox. I took a look back through my history, and noticed my Cron emails had also stopped being sent, with the last one arriving just before 2:30 EST yesterday. This is what I saw when I looked at the mail logs in /var/log/mail.err:
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Mar 30 08:05:06 server sSMTP: Invalid response: 501 5.5.4 HELO/EHLO argument email@example.com invalid, closing connection. ez6sm5598611pab.12 - gsmtp (firstname.lastname@example.org)
Mar 30 08:05:06 server sSMTP: Cannot open smtp.gmail.com:587
At first I thought it was Gmail somehow flagging my account as a spammer or something, but I noticed some of my FlexGet emails still went through... Then I decided maybe Gmail was suddenly rejecting usernames that included the @gmail.com part, but still no dice after I changed that. So maybe it was the server or port I was connecting to? Nope... But then... Then I struck gold!
It seems that Gmail is now filtering out special characters from the hostname we provide through SSMTP! I changed this to "email.address" instead of "email@example.com", and it works perfectly!
Hopefully this helps everyone else who happens to come across this little problem.