Mail servers started hanging up mid-stream on my outgoing qmail sessions.
To bring up a working system while I figure out what the hell happened, I deployed opensmtpd and rspamd.
It's a code increase of several orders of magnitude and a massive decrease in functionality. No wonder nobody wants to host their own mail server any more; if you're not using qmail, the tools are downright crippled.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko