Got it!!!
If I add smtp_sasl_security_options = noanonymous in top of main.cf it works.
I added the line to /etc/e-smith/templates-custom/etc/postfix/main.cf/90smarthosts_accounts and did an signal-event nethserver-mail-common-save.
It is still working. Hopefully the credentials are not cached. -> It does
What is the provider that does not support TLS? If some provider does not support TLS+SMTPAUTH on 25, I suggest adding port :587 to relaymaps.
The postfix manual says this is an insecure approach. I think we should require encryption in tls_policy, by replacing none with encrypt.
Sorry for confusing. I got it working with encryption enabled 
But the setting smtp_sasl_security_options = noanonymous is still needed. Web.de and gmx.de only allow plaintext authentication covered by a tls connection.
Happy new year @All
The configuration is now working since 1,5 weeks.
I am very happy with that. Is there anything I can do?
Will this special config being integrated into the standard config?
It would be great to summarize this discussion and write down an howto 
I really don’t know. It’s quite complex so it depends on how many people will ask for it.
You’re ready to create a great howto on our wiki! http://wiki.nethserver.org
Our docs team @AbsyntH, @sitz, @Ctek, @stephdl, @WillZen, @Jim will definitely help you through the job
I am asking for it.
@Linux4All great job. Thank you. This was one of my task on my todo list.
I will help to create a documentation. Give me some time please.
Now you’re talking! 
Please keep us in touch, I’m keen to know how to sort it out.
Hey my friends,
please give me some hints how I should write the documentation.
Which format, document type, etc.
Imho it should be added to the official documentation under http://docs.nethserver.org/en/latest/
In the meantime you can add a new page to our wiki http://wiki.NethServer.org and ask here advice. Right @stephdl?
yes, go ahead
Hi @Linux4All, just take a look at this http://wiki.nethserver.org/doku.php?id=howto:tips_to_create_how-to_s
Just some general tips.
If you have more questions we are here to help you.
Did you have created a new doc yet? Keep us in touch
I will do it the next days 
@Ctek
As promised I have started a howto. The dokuwiki syntax is hating me 
Perhaps you can help me?
Something is going crazy at the following lines.
vi /etc/e-smith/templates-custom/postfix/postmap/40smarthosts-accounts
If you change the “-” to an “_” from 40smarthosts-accounts all further formatting is lost.
Some lines before is nearly exactly the same text without any problems.
Why?
Here is the link http://wiki.nethserver.org/doku.php?id=howto:how_to_configure_nethserver_for_mail_configuration_with_office365_multiple_mailaccounts
Hi, I’ve solved the issue, and congratulations for your hard work!!
Tip:
Try to use CODE tag instead of FILE 
Also try to start the row from the begining without spaces when you can
for example in this paragraph:
{
push @postmapList, qw(relaymaps);
’’;
}
I have tried a lot of variations, nothing was successful. -> Thank you for that!
If you click on the button insert code -> you get “file” instead of “code” -> 
I know…
This is why i write manually the tags 
Yeahhh! It’s done. Here it is
Have fun!
Would be great if somebody could read it/try it and give me some feedback.
A link in the Admin Manual would also be great!