Quoting from the "solution":
It is rare, but it is possible that the server will use a different
certificate on different ports, so switching between 465, 587 and 25 might yield better results. Again, the best source for the settings is your email provider.
I tested both 465 SSL and 587 TLS right after encountering the solution, and they both work, though I am under the impression that Gmail favours TLS (whereas I've hitherto always been using SSL). Any thoughts on that?
Hartmut W Sager - Tel +1-204-339-8331
On Mon, 20 Apr 2020 at 10:47, Gilbert E. Detilllieux < gedetil@cs.umanitoba.ca> wrote:
Initially, it was just a guess, based on the reported error message. Then I started to suspect that was the problem. (Knowing that the host names used in the config were in the SAN list, and that the canonical name wasn't.) Then, one of our grad students found these forum posts...
Issue: https://support.google.com/mail/thread/38789651?hl=en Recommended answer: https://support.google.com/mail/thread/38336515?msgid=39890656
... which confirmed my suspicions. Adding the PTR record fixed the problem, and further confirmed my suspicions and what was claimed in the forum posts.
Incidentally, cranking the sendmail LogLevel past 11 (and all the way up to 14) didn't shed any extra light. All we saw was that connections from google.com servers would start TLS negotiations, and then just disconnect before issuing any SMTP commands. I had to rely on users to send me error messages from their bounce messages to see what was failing on the client side, i.e. Google's side:
TLS Negotiation failed, the certificate doesn't match the host.
Not much to go on!
Gilbert
On 2020-04-20 10:03 a.m., Hartmut W Sager wrote:
How did you (and your work colleagues) come to realize that the RDNS name is what Google cert is trying to match?
Hartmut W Sager - Tel +1-204-339-8331
On Mon, 20 Apr 2020 at 09:20, Gilbert E. Detilllieux <gedetil@cs.umanitoba.ca mailto:gedetil@cs.umanitoba.ca> wrote:
Yeah, we got bitten by this one at work last week (actually started noticing problems around Wednesday or Thursday of the previous week, but only resolved it last week). We had some users within the department who wanted to manage their CS e-mail via Gmail, and had set up their Gmail to send out via our SMTP server, which had worked fine until Google unilaterally decided on this new restriction, which mostlikely
violates several standards. Since we control reverse DNS on our domain, I was able to fudgethings
up to get Google to accept our cert (a legitimate cert, issued by Globalsign, that has multiple generic aliases in the SAN list, but intentionally avoided the canonical host name, since these generic aliases should be allowed to migrate to different physical servers, transparently). I found out that you can have multiple PTR recordson
one IP address, which is completely legal in DNS, but not usually considered good practice (or so I thought). Of course, this secondPTR
record caused some things to fail in a non-deterministic way, since lookups on the IP address gave the PTR records in pseudo-randomorder,
causing code that only looked at the first answer to get inconsistent results. Grrr! Thanks, Google, for once again messing with standards, and forcing everyone else to bend to your will! Gilbert On 2020-04-18 3:14 p.m., Hartmut W Sager wrote: [... deleted ...]-- Gilbert E. Detillieux E-mail: gedetil@cs.umanitoba.ca Dept. of Computer Science Web: http://www.cs.umanitoba.ca/~gedetil/ University of Manitoba Phone: (204)474-8161 Winnipeg MB CANADA R3T 2N2 Fax: (204)474-7609