RainLoop v1.11.3 App integrated in Nextcloud 12.0.4
System is running on Synology NAS using: Apache 2.4, PHP 7.0
Mail Server v1.6.4 from Synology on same machine (via localhost reachable)

I can't send mails, when SMTP-encryption (SSL/TLS) is used.
Error: Authentification failed
no matter what I choose:

• SSL 465 or STARTTLS 587 or 25
• localhost / IP / domain

only the way SMTP without encryption and without authentication works !

"other mail-clients" on the same machine (connect via localhost),
or apps on mobilephone (connect via domain) ect,
with the same login-data, works fine !!

the challenge is, that the mail-server request a username, not mail-address !
so I select the option: "use short usernames"

using IMAP with SSL and sending mails without SSL works also fine with my settings.
I think there is a problem with "use short username" on SMTP
so the mail-server doesn't get the correct login for authentication.

so please help me
thanks :)

1st picture shows the error message when SSL is enabled

2nd picture the only granted settings - but not using SSL/TLS !

3rd picture is a sample from Windows10 Mail-App with working settings.