I've seen it too.
Running under macOS 12.4.
For Mail I have:
iCloud: imap with p35-imap.mail.me.com with SSL on for both incoming and outgoing servers.
Two Verizon.net accounts, both with SSL on and imap.aol.com for incoming and smtp.aol.com for outgoing
gmail: imap.gmail.com & SSL on for incoming, and p35-smtp.mail.me.com & SSL on for outgoing
Also quite a few folders and sub-folders On My Mac.
In addition I have Thunderbird running with:
Verizon.net: imap.aol.com with SSL on and OAuth for incoming and smtp.verizon.net with SSL on and OAuth for outgoing
gmail: imap.gmail.com with SSL on and OAuth2 for incoming and smtp.gmail.com with SSL on and OAuth2 for outgoing
Perhaps:
For the gmail account in Mail I shouldn't be using an iCloud server for outgoing
For the Thunderbird accounts I should stop using Verizon servers mixed with AOL servers. Verizon went through a messy transition a few years ago and its email was handed off to AOL. I suspect what I have now was working during the transition and I've left it alone since then.