Outlook 2003 worked fine, upgraded to 2007 and got authentication errors on send (e.g. error from Outlook, 0×800CCC0F unable to communicate with server), using smtp outgoing and imap for receive.
Solution: Change smtp port from 25 to 587(SMTP submission port)
But there is another SMTP submission port, port 587, which almost all mail server supports. In fact, according to the relevant standard, port 587 is the preferred port for mail submission.
...If your talking about bulk mail delivery, port 587 it's not used for that. It's used normally for personal one to one email.
ISP's used to allow you to use port 25 on their mail server or mail servers that are not hosted on their network until the virus and spam issue exploded a few years back. So today, some Internet service providers have blocked access to SMTP port 25 for sending mail OUTSIDE of their network to curb viruses and spam when they try to send email from your computer at home. This means you can use their mail server (their email accounts) to send email, but if you 0wn3d (owned) your own domain and mail server outside of their control, you can't send email to it on normal port 25. You have to use port 587 which is authenticated (hopefully since you set it up) to send email to it and then that server will deliver it to the receiptant via port 25.
Another example of using it is some hotels and other free Wi-Fi (Starbucks) also hijack (sniff/watch) port 25 connections leaving their network so they can check for spam or other virus laden issues. When those issues happen it's their network that ends up in trouble. If you don't like them reading your email, using port 587 to your mail server. They are of the same mind set that port 25 is to open and un-authenticated to send email on from their network,
See, it's understood today that email clients (Outlook) or computers shouldn't be sending email in port 25. A mail server sends email on that port. We are in essence saying mail servers will send email on port 25 to each other, but when submitting a message to your mail server for delivery to someone else or to a system not on the same network, use port 587...