Pb with email configuration with dovecot

Hello everybody
I'm trying to resolve my email issue for many days. I searched many forums and can't find any solution
My configuration is: webmin/virtualmin v 1.730, with postfix v 2.7.1 and Dovecot v 1.2.15
My problem is with Dovecot (I think)
I created a user/email account and try to configure on thunderbird
I can receive email, but I can not send (I precise that I can send from webmin admin )
My error on thunderbird :
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE STARTTLS AUTH=PLAIN] Dovecot ready
I am not very familiar with dovecot, I just want to have email account and ability to send
If anybody can tell me how resolve dovecat's problem or another way to have email account please tell me
Thanks