"Without information we are helpless and just guessing".
David, I was getting the error message below, since I installed Avast 5.....
An unknown error has occurred. Account: 'pop.gmail.com', Server: 'smtp.gmail.com', Protocol: SMTP, Server Response: '421 Cannot establish SSL with SMTP server 74.125.77.109:25, SSL_connect error 5', Port: 25, Secure(SSL): Yes, Server Error: 421, Error Number: 0x800CCC67
I did as you said and unticked the encryption box in Avast Real-Time Shields, Mail Shield, Expert Settings, SSL Accounts, Encryption column and set the option to "None"
everything works fine. Select "SSL" and the error message appears for outgoing mail.
But my gmail settings have the "SSL" box ticked.
I am wondering if this makes sense ...