The standard port used for SMTP connections is SMTP Port 25.

SiteGround too offers alternative SMTP ports – 2525, 587. For secure SMTP over SSL you can use  port 465.

How are these ports different?

SMTP Port 25:

Port 25 is the oldest of the four. It was the port number assigned to SMTP when the protocol was first introduced in the now obsolete RFC 821 back in 1982, about 37 years ago.

This SMTP port remains to be used primarily for SMTP relaying. SMTP relaying is the transmittal of email from the email server to the email server.

Several ISPs, however, block port 25, that’s why we suggest using one of the alternative SMTP ports.