Outlook 2007 - Can't send e-mail (receiving OK)

I am having a new problem with Outlook 2007. I am unable to send outgoing messages. I have tested it across all three Pop-3 accounts that I use and the problem is common to all of them. Incoming messages are coming through just fine. I know that my UserID and password are OK for all accounts as I am able to get in through WebMail. I have tried the following to date with no success:

1) Restarted my machine
2) Deleted and recreated my e-mail accounts
3) Turned off all of my virus protection/firewall/spam filters (Norton 360)
4) Went back to the last restore point, which was before the problem started to occur

The only other piece of information that might be useful is the evening this started I got a very brief e-mail message that said I couldn't access my e-mail (Outlook was running at the time) because a back-up was in progress. The message disappeared before I could copy it. I have run Malwarebytes on the whole machine just to check for viruses and found nothing.

I am running Windows 7 as my OS. Any suggestions are much appreciated.

Maybe your outgoing mail server requires authentication.

In Outlook: Tools ==> Account Settings ==> Click the account ==> Change ==> More Settings ==> Outgoing server" ==> Select "Outgoing server needs authentication" and "Use same settings as my incoming mail server".

In your account settings you will also find a button "Test Account Setting" which will immediately show you if this fix helped you.

Thanks for responding. Turns out that Verizon (I use FIOS service) blocked Port 25 for my outgoing e-mail, which is the default port used the company that hosts my mail service. I switched to the back-up and it works like a champ. I guess this is fairly common by the larger ISPs. Thanks again for your help.