"外寄邮件伺服器" can be translated into English as "Outgoing Mail Server" (or more formally, "SMTP (Simple Mail Transfer Protocol) Server" when referring to the technical protocol behind it).
Outgoing Mail Server is the common and user-friendly term used in email settings and general contexts.
SMTP Server is the technical term that specifies the protocol used for sending emails.
In most cases, "Outgoing Mail Server" would be the appropriate choice for everyday use.