Explain E-mail gateways.
Email using SMTP works better while both the sender and the receiver are on the Internet and can support TCP connections among sender and receiver. Though, many machines which are not on the internet still need to send and receive emails from internet sites.
The other problem occurs while the sender speaks only RFC 822 and the receiver speaks only X.400 or several vendor exact protocols.
Both of such problems are solved using email gateways. Email gateways are utilized at application layer.