balsa : sending msgs impossible

[1two free fr]

Hi, first of all, thx for the great work you did inventing a light and easy mail client.

For some reason, I cannot send out emails, though. I have looked for solutions, none of the proposed worked 
so I have to ask you.
I use Balsa in Antergos.

My outgoing server is set to smtp.xxxx.com. Recently I added :25 or :smtp at the end because i read this 
thread:

Why do I get "SMTP server problem: SMTP Server dropped connection"?
    Balsa's default is to use the "submission" service on port 587. But most servers still use the SMTP 
protocol on port 25, so just put ":smtp" or ":25" behind the server, ie. "smtp.addcom.de:25".

I have tried the settings with and without user name and/or pw, I have erased the server profile, 
reconfigured it - nothing works. It is impossible to send an email. Everytime I get an error msg and the 
email stays in the outbox.

Note that on my iPad and on my Android tablet I use simply smtp.xxxx.com without user name and pw and on 
these I CAN send out emails.

I can't seem to find the solution so I will be very thankful if one of you has a clue.

Thanks again for your precious help.