Copying my response here in case there are others with same issue (and I expect there will be).
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Thank you for the log.
All of a sudden, the outgoing mail server, smtp.mac.com, does not "want" to enter STARTTLS (encrypted) mode.
I know it's worked fine for a long time, and the settings in AquaMail (smtp.mac.com / port 587 / STARTTLS) are from Apple's support site.
I can reproduce the issue without any mail apps at all, using the command line "openssl" tool in Linux:
- iCloud accounts (@me.com / @icloud.com) use smtp.mail.me.com:
$ openssl s_client -crlf -starttls smtp -connect smtp.mail.me.com:587
...
SSL-Session:
Protocol : TLSv1.2
Cipher : AES128-GCM-SHA256
- @mac.com accounts use a different server name, smtp.mac.com:
$ openssl s_client -crlf -starttls smtp -connect smtp.mac.com:587
...
139950998923128:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake failure:s3_pkt.c:617:
So they (Apple) messed something up on the server side.
You may want to try turning off STARTTLS (encryption). I can only assume that this is how your desktop mail app is set up, since it continues to work.
Long press the account -> account setup -> Manual -> Next
You'll be looking at detailed outgoing server settings
Change "encryption" from "STARTTLS" to "None", press Next to try and validate.
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Reporting the issue to Apple support and including the openssl snippets from above should help too (I hope).