SMTP refuses to send messages

[Francisco Ortiz Santini <ortizsantini prtc net>]

Hello:
 I am in the process of switching from Evolution to Balsa.
However, Balsa refuses to send messages. I have tried every suggestion I
have found here and elsewhere. Here are the details.
 I am using RedHat 9 on a HP Pavilion a265c. I have used Balsa 2.0.6 and 2.0.12, with
libesmtp 0.8.12-5. At first (using Balsa 2.0.12) the send function
waited for about 2 minutes before failing, stating that the smtp
provider failed to respond. More recently, and using either Balsa 2.0.6
or 2.0.12 I just receive a message that says "SMTP server problem (2):
Nothing to do. Message is left in outbox" or it simply stays idle until
failing. My mail service provider is "mail.prtc.net", either in POP and
SMTP. I tried adding ":smtp" and ":25" in the settings, but nothing
works. The provider is working as usual and I can send mails in
Evolution and sylpheed using exactly the same settings. I do not know
what else I can do, so please help me. Every suggestion will be greatly
appreciated.
Francisco