SMTP relay server
As a smart host , mail relay server or smart host is a mail server ( B hereinafter), which from a transmitter ( A ) E-mail accepts and to any third parties ( C forwards).
- A (sender) → B (relay server) → C (receiver)
A correctly configured SMTP relay server (smart host) only forwards e-mails if it
- either mails of A is responsible
- Example: a customer ( A ) can send mails to any recipient ( C ) via the mail server of his provider .
- and / or mail at C is responsible
- Example: The mail server of a company ( C ) receives mail from any senders ( A ).
An insecurely configured SMTP relay server is a so-called open mail relay and also forwards e-mails for which it is not responsible for either A or C. Such mail servers are misused by spam senders to send massive amounts of spam and are then often put on blacklists such as DNSBLs . Not only actual mail servers, but also members of botnets can represent open relays.
commitment
Relay server for e-mail programs
SMTP relay servers are mainly used by e-mail programs to send e-mails. In certain cases a registration is necessary, cf. SMTP-Auth and SMTP-After-POP .
Relay server for mail servers with dynamic IP
Mail servers with dynamic IP addresses are not allowed to deliver to many recipients, the sent e-mails run the risk of being filtered out by spam filters . To avoid these problems, the sender can use a relay server that the recipient considers to be trustworthy. As a rule, the sender must authenticate himself to the relay server before using it , cf. plus SMTP-Auth and SMTP-After-POP.
Special-purpose relay server
Offers for online spam filters and online virus filters are usually implemented with relay servers. Before the e-mail is delivered to the actual recipient's mail server, it goes through the relay server and is filtered there.