Some providers block outgoing SMTP ports by default and charge to unlock them.
I solved it by creating an SSH tunnel to an unrestricted machine, but this leads to an invalid certificate error in ItFlow, since it connects to 127.0.0.1 on a given port, but the certificate it gets is actually from, for example, Google.
To work around this, I added a static host entry for the target SMTP pointing to localhost, and that way it works fine.
It wouldn't be a bad idea to have an option in the email configuration to ignore invalid SSL certificates and proceed with sending.