This might be a clue - TRACEROUTE shows packet loss en route to Gmail via BT Broadband - here's a log I did just now:
In particular look at: 18.104.22.168 - problems with this server have been reported online before.
traceroute: Warning: smtp.gmail.com has multiple addresses; using 22.214.171.124
traceroute to gmail-smtp-msa.l.google.com (126.96.36.199), 64 hops max, 52 byte packets
1 192.168.2.1 (192.168.2.1) 1.045 ms 0.668 ms 0.725 ms
2 188.8.131.52 (184.108.40.206) 12.821 ms 12.758 ms 12.844 ms
3 220.127.116.11 (18.104.22.168) 96.947 ms 11.697 ms 23.156 ms
4 22.214.171.124 (126.96.36.199) 38.743 ms 113.834 ms 12.869 ms
5 188.8.131.52 (184.108.40.206) 12.610 ms 12.853 ms 12.562 ms
6 220.127.116.11 (18.104.22.168) 12.795 ms 13.285 ms 12.057 ms
7 acc1-10gige-0-5-0-5.bm.21cn-ipp.bt.net (22.214.171.124) 18.557 ms 12.656 ms 12.262 ms
8 core1-te0-15-0-17.ealing.ukcore.bt.net (126.96.36.199) 24.255 ms
core1-te0-15-0-16.ealing.ukcore.bt.net (188.8.131.52) 20.803 ms
core1-te0-15-0-15.ealing.ukcore.bt.net (184.108.40.206) 21.090 ms
9 peer1-xe0-0-0.telehouse.ukcore.bt.net (220.127.116.11) 18.080 ms
peer1-xe1-1-0.telehouse.ukcore.bt.net (18.104.22.168) 19.330 ms 20.231 ms
10 22.214.171.124 (126.96.36.199) 19.960 ms 17.739 ms
188.8.131.52 (184.108.40.206) 26.123 ms
11 220.127.116.11 (18.104.22.168) 44.810 ms
22.214.171.124 (126.96.36.199) 68.025 ms 23.325 ms
12 188.8.131.52 (184.108.40.206) 22.239 ms 21.646 ms 39.272 ms
13 220.127.116.11 (18.104.22.168) 37.278 ms 24.102 ms 23.611 ms
14 22.214.171.124 (126.96.36.199) 22.716 ms 22.932 ms 22.713 ms
15 * * *
16 wg-in-f108.1e100.net (188.8.131.52) 23.544 ms 23.503 ms 23.070 ms
You are all getting hung up in irrelivant details. This is a bug in mail.app where (for a period) it will not send authentication information to a remote mail server. Its not isolated to any particular mail host or ISP. It starts working again by itself if nothing is done so anyone claiming to have a solution, doesnt. It can and probably will happen to them again no matter how often they re-add details to their setup or remove certs from keychains.
Apple needs to look at this although it does seem to have been more stable recently.
Please learn about networking IF you are going to make statements like that.
You have not prooved any packet loss at all. All you have done is send out some ICMP echo packets and waiting for replies and timing them. Since most routers will prioritse other packet/functionss over ICMP echo replies, they often come up as high round trip times or even missing but that does not mean there is anything wrong with that route at all. If packets were not getting to gmail then being TCP you would get TCP resends or broken connections which is a different issue and you would see with more than just smtp.
Just because YOU have this issue on BT when connecting to gmail does not mean its isolated to that. I have had it a few times as have a few of my customers and none of us use BT nor gmail. I have done extensive debug tracing on both the local and remote server ends and isolated the lack of authentication as the issue. Durring the investigation, it suddenly started working again as mail.app decided to start passing authentication information with SMTP outbound requersts. Further analisys of logs from other incidents confirmed that authentication was lacking on other occasions from mail.app.
Read the title. Its SMTP outgoing issues since ML upgrade not "I have a problem with my BT connection and think its fun to polute my useless diagnostics to people investigating a different fault just because I have not been sufficiuently beaten with the clue stick this week".
Try a BT forum not an apple one.
Here's another thing you've missed.
Failure to send via Mail can be due to specific email content e.g. Evernote Support emails...
I once had 7 emails queued and they only went once I'd deleted a specific email.
Is this in the thread? no... Just you being fascist about the cause...
Bad Karma? Says it all...