Normally when you send email, the email will be queued by the FastMail.FM server. From there, we find the appropriate server for each recipient of the email, and forward it on to that server directly.
However in some cases, you actually want to send the email via another server using authenticated SMTP, such as your ISP or corporate email server. This is especially useful if you have a corporate email server that requires SMTP authentication to send to particular internal mailing lists, or you want to avoid SPF issues.
Using the new External server/External username/External password options that can be set on the Options –> Personalities screen, you can send email via an external server with SMTP AUTH. If enabled, we will try and send using SMTP AUTH via port 587, the standard SMTP submission port. If the remote server advertises STARTTLS support, we will try and switch to TLS on the connection. If you want to use a different port, you can append :port-number to the server name. Unfortunately straight SSL over SMTP (port 465) is NOT currently supported.
This feature will also be used if you send email via FastMail’s SMTP server if the From header email address matches a corresponding personality email address and the Use external SMTP server option is set.
More information about this feature and it’s use is in the personality help pages.