GMail SMTP - Strange behaviour

[Antonio De Benedetti]

>From at least one month I can't send email with Webmail Extesion ("Sendinf 
of message failed....").
I discovered that configuring Thunderbird "Outgoing Server (SMTP) Settings" 
with another account I can send emails without any problem.
Why? Have you any idea?
If it can help, the not working account is an old one (at least 3 year 
old), and the working account is newer (about 1 year old).