I'm still playing with my VestaCP install, and I've found that the outgoing mail doesn't work correctly when using Thunderbird or any other mail client except webmail which works perfectly.
It turns out there's an issue with the SSL certificates and EXIM (the mail server)
Here's what I did to fix it, just make sure you replace the example with your own domain. If you aren't running as root, use these commands otherwise put sudo in front of them all:
ln -s /home/admin/conf/web/ssl.example.com.pem /usr/local/vesta/ssl/certificate.crt ln -s /home/admin/conf/web/ssl.example.com.key /usr/local/vesta/ssl/certificate.key setfacl -m user:Debian-exim:r-- /home/admin/conf/web/ssl.example.com.pem setfacl -m user:Debian-exim:r-- /home/admin/conf/web/ssl.example.com.key chgrp mail /home/admin/conf/web/ssl.example.com.pem chmod 660 /home/admin/conf/web/ssl.example.com.pem chgrp mail /home/admin/conf/web/ssl.example.com.key chmod 660 /home/admin/conf/web/ssl.example.com.key
Now you should be able to send email from a mail client without it complaining about certificates :)