I've done a lot of work with Net::SMTP on several platform and have found the performance to be comporable on roughly equivalent hardware regardless of OS. Perhaps if you gave us some details on your setup we could help: are you using persistant SMTP connection, how is postfix configured, is postfix running on thesame server with your perl script (i.e. was postfix also moved from Solaris to Linux), what hardware were you using for the Solaris and Linux setups, etc...