Widely used program that implements the SMTP mail delivery protocol on most UNIX and Linux systems. If someone’s ISP delivers email using SMTP, it is important to configure sendmail correctly to avoid “bouncing” email. If sendmail does not know a particular user name, it will reject the email and deliver the error message “550 User unknown.” As with regular land mail, when a recipient is not known because of a wrong or changed address, the land mail will be returned to the sender. The same principle applies to email. Bouncing wanted email is considered to be a beginner system administrator’s mistake by more seasoned experts, especially when it is from a mailing list.

Bouncing wanted email can occur when connecting UNIX to the Internet for the first time. These techniques can increase the chances that correctly addressed email is accepted by sendmail. Make sure that: any user name to which email is addressed is defined as a UNIX user, any name used on email is defined as an alias to UNIX users, and email addressed to unknown user names is redirected to defined UNIX users.

See Also: Electronic Mail or Email; Internet Service Provider (ISP); Simple Mail Transfer Protocol (SMTP); UNIX.