I wish to use Torbirdy alongside usual GPG encryption for emails. But I read, to prevent spam, that all exit nodes block SMTP traffic usually. So how does Torbirdy addon for thunderbird ensure that emails are exiting from exit node?
Note that by default, TorBirdy will configure your email account to use port 465 and not port 25 for the SMTP server. This in fact should be your default setting: you should not be using port 25 for your email over Tor as your username and password are sent in clear text and can be sniffed by the exit node.
Most providers use 465 for
SSL/TLS and 587 for
STARTTLS and this is what is recommended for email over Tor. You can change this port to what your email provider recommends, but if your email provider supports only plain SMTP over port 25 without
STARTTLS, you should find a better one and not use it over Tor :)
There are some relays which allow exiting at port 25. Furthermore several mail servers don't accept mail on port 25, but use mail submission on port 587. At the moment of writing there are 131 relays which explicitly allow port 587 and 11 which allow exiting on port 25.