There have been times when I’ve been travelling and I’ve been unable to send email to the POP email account that I have in Outlook 2010. After some research, I found that the cause of this was that not every ISP was supporting the standard TCP port 25 for sending POP email. In an effort to reduce spam some Internet providers like COMCAST changed from port 25 to port 587.
To change the port, select the File tab then select Account Settings to display Outlook’s Account Settings dialog. Next select the POP account that you are having trouble sending email from and click Change. Next, click More Settings and then select the Advanced tab to display the Internet E-Mail Settings dialog that you can see in Figure 1.
Change the value in the Outgoing server (SMTP) prompt from 25 to 587 as is shown in Figure 1 and then click OK. Most Internet providers support sending email on port 553 so this should address the problem.