A common feature of SMTP servers is open-relaying. Open-relaying allows anybody on the internet to relay an email message through an SMTP server. The disadvantage of open-relaying is that it makes all messages appear to originate from the relaying server. For this reason, open-relaying is usually disabled because a spammer could use an SMTP server with an open-relay to send spam, and it would appear that the owner of the SMTP server is sending the spam instead of the spammer. This makes it very difficult to trace the origin of the spam and stop it.
If a Bell customer were to operate their own SMTP server, it is possible that it may be configured improperly with open-relaying enabled, and have spam relayed through it. In this situation, it appears that a Bell customer is sending the spam. So Bell avoids this situation by ensuring that no customer can operate their own SMTP server, and blocking port 25 effectively does just that.
One of the disadvantages of this arrangement is that it also prevents the use of any port 25 SMTP server other than the ones that Bell provides. (SMTP servers that use a port other than 25, like GMail which uses 465 for example, are not affected by this block.) So if you have a third-party e-mail provider, you will not be able to send outgoing mail through their SMTP server via port 25. You must use a Bell SMTP server over port 25.
Another disadvantage is that it prevents a Bell customer from running their own incoming SMTP e-mail server.
Please note that port 25 blocking does not affect incoming email through POP3 servers, which do not use port 25. You are free to use whatever POP3 provider you wish.
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