smtp internal mail server problem

Hi, I’m new user of Em-Client, and it seems very nice, however I have two problems connected with the sending of email to internal mail server.
We have an internal server, at address where each user has an account of internal mail like xxx@postainterna.
-Problem 1) if I set the smtp server in the following way (which works in thunderbird, that I don’t like) server:
        port: 25
        security policy: do not use non secure connection
        authentication: checked, the same of the identity
I obtain a ‘Failed’ result when I try to run the diagnostic

-Problem 2) if I try to insert an internal recipient, like ‘yyy@postainterna’ I obtain a message telling this is not a valid address.

How I can solve?

Obviously I checked any possible combination of authentication/port/username(with or without @postainterna), but the results are always the same.

Same configuration is perfectly working in Thunderbird, and POP3 is working too.

Thanks in advance to anyone able in helping me with this issue.