All Smtp
methods are thread-safe in the sense that they fail if called while another method is in progress.
Reusing the same Smtp
session is possible (and useful if you need to send more messages through one SMTP server), but you have to keep track of whether a method is currently active on a specific Smtp instance yourself.
However, please note that when connected to third-party SMTP servers, keeping connections idle unnecessarily (until the service stops or until they fail) would not be a good practice.