On Thu, Jan 13, 2022 at 05:25:41PM +0000, Stuart Henderson wrote: > On 2022/01/13 18:05, Leo Unglaub wrote: > > Hey, > > > > On 11/01/2022 21:28, Stuart Henderson wrote: > > > I bet it is MTU related. Try lowering MTU on that interface (you > > > cannot do it separately for IPv4 and IPv6 so it will change both, > > > but that's not likely to be a problem) and get someone who has > > > seen the problems to re-test. > > > > thank you so much for your answer. I would have never ever thought about the > > MTU in this case. I used the default 1500. I talked to the technical support > > from the datacenter (Hetzner Online) and they asured me that 1500 is > > correct. > > > > However, i have set the value to 1400 and asked some people who had the > > issue to re-test it. I will post the results of the test here so other > > people can find them via a search engine. > > > > Thank you so much, very kind of you! > > The possible issue is that many people (especially people connecting > over tunnels, but also those on pppoe) are on lower MTUs than this. > Normally this is OK as fragmentation-needed messages will sort things > out but sometimes firewalls are not be configured to pass these which > will cause problems. If that _is_ what's happening then there are > other ways to fix it but changing MTU is often the easiest one that > you can do yourself.
Well, I can connect to his server using: openssl s_client -starttls smtp -connect mail.unglaub.at:25 The handshake completes and I'm able to issue smtp commands. However smtpd always reports that opportunistic TLS failed, and downgrades to plaintext.
