Registry SMTP Servers define configurations for relaying outgoing email.
Configuration
SQL Servers are added via the Server Registry, ie ALL > Servers > 
Add a New Server. Plugin should be set to CoreServer.SmtpServers.
A hostname and port are required. The default port of 587 is usually the best choice, but this may vary according to the SMTP service provider. The use of TLS is generally recommended.
A username and password are typically required, though some servers may permit sending mail on a per-IP address basis, in which case these fields can be left blank.
A default from and reply-to address are required, though it may be possible to override these under certain circumstances. (For example, Message Templates permit per-Template reply-to addresses.) Note that many mail relay services will overwrite the from address if it isn't associated with the relay account.
A Delivery Override address may be specified. see Overriding Delivery For Testing for more information.