知道为什么了。Bell Sympatico 堵塞了Port 25，为了反垃圾邮件。
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Question: Why does Sympatico block all SMTP servers except its own? (Port 25 blocking) (#8457)
Answer: Sympatico blocks all traffic (incoming and outgoing) through port 25, except for it's own SMTP servers. The main reason that Sympatico does this is to avoid being being blacklisted for spam (junk e-mail). Here's why:
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 Sympatico 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 Sympatico customer is sending the spam. So Sympatico 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 SMTP server other than the ones that Sympatico provides. So if you have a third-party e-mail provider, you will not be able to send outgoing mail through their SMTP server. You must use a Sympatico SMTP server. Your outgoing mail will still appear with whatever e-mail address you wish, even if it is not a Sympatico e-mail address.
Another disadvantage is that it prevents a Sympatico 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.
