If you want to force your site to use a specific SMTP server instead of the system one, enter its hostname here.
It is possible to specify more than one host by separating them with semicolons, e.g. smtp1.example.com;smtp2.example.com, but keep in mind that all other settings (e.g. authentication credentials and port numbers) will apply to all listed servers. It is not possible to specify different credentials for each server in this list. This feature is useful when SMTP host authentication is not required or you list different frontends for the same mail server in which case other settings will work.
This field may be disabled if overridden by a setting in your config.php file.