What error messages do you get in your sogo.log?

This is the log from starting a session to writing the message.

...
Mar 22 11:47:43 sogod [2406]: 10.1.0.254 "GET /SOGo HTTP/1.1" 302 0/0 0.002 - - 0
Mar 22 11:47:43 sogod [2406]: 10.1.0.254 "GET /SOGo/ HTTP/1.1" 200 7437/0 0.018 27414 72% 0
Mar 22 11:47:47 sogod [2406]: SOGoRootPage successful login from '10.1.0.254' for user 'user' - expire = -1 grace = -1
Mar 22 11:47:47 sogod [2406]: 10.1.0.254 "POST /SOGo/connect HTTP/1.1" 200 48/58 0.072 - - 0
Mar 22 11:47:48 sogod [2406]: 10.1.0.254 "GET /SOGo/user HTTP/1.1" 302 0/0 0.005 - - 0
Mar 22 11:47:48 sogod [2406]: 10.1.0.254 "GET /SOGo/user/view HTTP/1.1" 302 0/0 0.007 - - 192K
Mar 22 11:47:49 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Mail HTTP/1.1" 302 0/0 0.005 - - 0
Mar 22 11:47:49 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Mail/view HTTP/1.1" 200 19760/0 0.065 84173 76% 144K
Mar 22 11:47:50 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Calendar/alarmslist?browserTime=1553251670 HTTP/1.1" 200 429/0 0.033 - - 0
Mar 22 11:47:50 sogod [2407]: 10.1.0.254 "GET /SOGo/so/user/Calendar/paula_personal/A16267DD-779B-45C9-A88A-770849E1F1FC.ics/?resetAlarm=yes HTTP/1.1" 302 0/0 0.030 - - 0
Mar 22 11:47:50 sogod [2407]: 10.1.0.254 "GET /SOGo/so/user/Calendar/paula_personal/A16267DD-779B-45C9-A88A-770849E1F1FC.ics/view?resetAlarm=yes HTTP/1.1" 200 717/0 0.132 - - 0
Mar 22 11:47:51 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Mail/0/view HTTP/1.1" 200 77085/0 0.527 - - 4M
Mar 22 11:47:52 sogod [2406]: 10.1.0.254 "POST /SOGo/so/user/Mail/0/folderINBOX/view HTTP/1.1" 200 2334/48 0.695 - - 260K
Mar 22 11:47:53 sogod [2406]: 10.1.0.254 "POST /SOGo/so/user/Mail/unseenCount HTTP/1.1" 200 21/31 0.419 - - 0
Mar 22 11:48:04 sogod [2407]: 10.1.0.254 "GET /SOGo/so/user/Mail/UIxMailEditor HTTP/1.1" 200 2800/0 0.010 12824 78% 0
Mar 22 11:48:05 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Mail/0/compose HTTP/1.1" 201 78/0 0.333 - - 0
Mar 22 11:48:05 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Mail/0/folderDrafts/newDraft1553251685-1/edit HTTP/1.1" 200 82/0 0.337 - - 0
Mar 22 11:48:10 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.r HTTP/1.1" 200 342/0 0.017 - - 0
Mar 22 11:48:10 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rog HTTP/1.1" 200 344/0 0.009 - - 0
Mar 22 11:48:11 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner HTTP/1.1" 200 347/0 0.012 - - 0
Mar 22 11:48:11 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner@ HTTP/1.1" 200 348/0 0.016 - - 0
Mar 22 11:48:14 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner@ka HTTP/1.1" 200 45/0 0.016 - - 0
Mar 22 11:48:15 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner@kabsi HTTP/1.1" 200 48/0 0.015 - - 0
Mar 22 11:48:15 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner@kabsi. HTTP/1.1" 200 49/0 0.016 - - 0
Mar 22 11:48:15 sogod [2406]: 10.1.0.254 "GET /SOGo/so/user/Contacts/allContactSearch?search=w.rogner@kabsi.at HTTP/1.1" 200 51/0 0.016 - - 0
Mar 22 11:48:24 sogod [2406]: [WARN] <0x0x5614a17983f0[SOGoUserDefaults]> expected an NSString for 'SOGoMailComposeFontSize' (ignored)
Mar 22 11:48:24 sogod [2406]: 10.1.0.254 "POST /SOGo/so/user/Mail/0/folderDrafts/newDraft1553251685-1/send HTTP/1.1" 405 86/140 0.336 - - 64K

...

When pressing the send icon, the window turns from green to red and gives the error message.

No further log entry written.

In postfix there is no mail sent.

/var/log/mail.log
...
Mar 22 12:17:17 s4 dovecot: imap-login: Login: user=<user>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=6437, TLS, session=<8D1k/6yE6pp/AAAB>
Mar 22 12:17:17 s4 dovecot: imap(user): Logged out in=89 out=1026
Mar 22 12:17:17 s4 dovecot: imap-login: Login: user=<user>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=6440, TLS, session=<tfpp/6yE8pp/AAAB>
Mar 22 12:17:18 s4 dovecot: imap(user): Logged out in=89 out=1026
Mar 22 12:17:33 s4 dovecot: imap-login: Login: user=<user>, method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=6515, TLS, session=<2ENdAK2EoJt/AAAB>
Mar 22 12:17:34 s4 postfix/smtpd[6355]: connect from localhost[127.0.0.1]
Mar 22 12:17:34 s4 postfix/smtpd[6355]: disconnect from localhost[127.0.0.1] ehlo=1 quit=1 commands=2
Mar 22 12:17:34 s4 dovecot: imap(user): Logged out in=89 out=1026

I attached two screen shots for this

I have replaced username with "user" as it happens with different users

It looks like you set SOGoSMTPAuthenticationType. Fix your SASL authentication in Postfix or disable it in SOGo.

SOGoSMTPAuthenticationType = PLAIN is the default and currently the only option.

As I stated before: Sending did work (no issue with 4.0.6 and the first few 4.0.7 rollouts).

There was absolutely NO change in postfix and dovecot setup. SASL is set up and works fine. It's SOGo that is broken.

Your suggestion is not a solution that is technically viable.

Some more additions:

I just upgraded to the latest daily build (sogo_4.0.7.20190323-1_amd64.deb)

I also followed the hint of yours and disabled the SOGoSMTPAuthenticationType = PLAIN entry in sogo.conf.

Now sending works again.

This, however, implies that the documentation is misleading:
SOGo Installation Guide 4.0.7 p38:

D: SOGoSMTPAuthenticationType: Activate SMTP authentication and specifies which type is in use. Current, only PLAIN is supported and other values will be ignored.

I interpreted this as being the default (which it obviously is not).

So there are two settings: DEFAULT and PLAIN.

Maybe you want to extend this to be more specific.

Thanks for the hint which was helpful after all (and thanks for the software which - when operational - is great).

When you set SOGoSMTPAuthenticationType, it activates SMTP authentication and the only supported value is PLAIN. If you don't define this configuration parameter, SMTP authentication is disabled.