When running on a host with a IPv6 address and contacting to a IPv6 capable SMTP server, SOGo uses and invalid EHLO string.
In my case my mailserver runs exim4 4.94.2 and doesn't like SOGos EHLO at all, resulting in reject of the connection, see the following exim log excerpt:

SMTP<< EHLO 2001:9e8:33a2:1:809e:46ff:fe35:920c
SMTP>> 501 Syntactically invalid EHLO argument(s)
LOG: MAIN REJECT
rejected EHLO from [2001:9e8:33a2:1:809e:46ff:fe35:920c] I=[2a03:4000:29:68d:ff64:0:1:e10f]:465: syntactically invalid argument(s): 2001:9e8:33a2:1:809e:46ff:fe35:920c
SMTP<< QUIT

Indeed according to RFC 2821, chapter 4.1.3 the command has to be
EHLO [IPV6:2001:9e8:33a2:1:809e:46ff:fe35:920c]

Additionally SOGo also displays a missleading message to the user. It says
"cannot send message: (smtp) authentication failure"
which looks like wrong username/password. But code 501 means "Syntax error in parameters or arguments." So SOGo would do good to display a more exact message or even use what the server responds.