Port 3478 is a TCP port used for the Simple Mail Transfer Protocol (SMTP). SMTP is a communication protocol for electronic mail transmission. It is used to send and receive email messages between email servers.
Port 3478 is an important port because it allows email servers to communicate with each other and exchange email messages. Without this port, email would not be able to be sent or received. Port 3478 is also used by some email clients to send and receive email messages.