Cannot send email - problem with TLS

Installation
1

Just used /discourse-doctor and got the following error on mail test:

Not able to send email Net::ReadTimeout

Then I set DISCOURSE_SMTP_FORCE_TLS: true as recommended here: Cannot send email - problem with port 465 - #10 by schungx

But this time I got the following error:

TLS and STARTTLS are mutually exclusive

So, then I tried to test the TLS connection setup with openssl

openssl s_client -connect 127.0.0.1:465 -debug -msg

So that command failed as well. The connection was established but no negotiation at all.

CONNECTED(00000003)
2

The problem was with the mtu size of docker interface. I have a PPPoE connection which has less then 1500 bytes mtu size - so what I had to do is to set MSS clamping in iptables.

iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS  --clamp-mss-to-pmtu

More info about the solution can be found here: ssl - OpenSSL hangs at CONNECTED(00000003) - Stack Overflow