At a hotel which blocks port 25. Of course, there is no way to find their SMTP server to send email. The solution is that many mail servers also accept mail on port 26, so you either have to find the SMTP server that works on the network (good luck). Or reconfigure your client to use Port 26. On bluehost.com and also atlarge.net, port 26 works fine, so you have to go to the Mail/Preferences/Accounts/Outgoing SMTP Server and click on the combo box and select Edit Server List then click on the STMP server and the Advanced pane. Choose use custom port and type in 26. I normally also turn off SSL to make sure it basically works.
Many ISPs are blocking what is called “Port 25” which is the port used to send e-mail. They are doing this to cut down on the amount of spam that is sent from their networks.
All e-mail sent via the Internet is routed through the port 25, the channel used for communication between an e-mail client and an e-mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam.
Port 25 blocking allows ISPs to block spam sent out through their networks, but it tends to punish the innocent that have a need to send through e-mail servers other than those belonging to their ISP. The ISPs that block port 25 require their SMTP server to be used instead of the remote SMTP server or a SMTP server running on your computer.