Outlook 2007 won't send email

What type of mail server do you have?

POP/SMTP with comcast

do you have the proper user settings on the smtp authentication?

Clearly I do, as the email sent from within "Test Account Settings..." is sent properly.

Do you have any Anti-Virus software on your computer?
You may have outbound email scanning turned on and ths could be interfering with your outbound email.
If you do have Anti-Virus software, turn off outbound email scanning and press F9 (send/receive).  If your outbox empties, leave the scanning off.

Can you also describe your environment a little please.  Are you a standalone PC using POP3 / SMTP to send mail, or are you connected to a server and using Exchange in Cached mode etc.
Thanks
Alan

Disabling my antivirus program had no effect.
Yes, I have a standalone PC using POP3/SMTP with comcast.net.

Are you trying to send via comcast.net or via your company server?
When you test, are you external to the company or internal?

A message in the Outbox that is non-italic means it will not be sent until you open it and try to resend.

Do you receive any error messages, and can you look at the Send/Receive error. It should show in the bottom right of Outlook "Send/Receive Errors".

Can you create a new outlook profile and check if the issue persists (Dont delete the existing profile)?

This is a home computer, using only comcast.net.
I understand about non-italic messages. Opening and sending again doesn't change this.
I get no Send/Receive errors.
I created a new account. A message sent from "Test Account Settings..." is successful. Any message created from "New" appears in my Outbox in non-italic font, and is not sent.
Thanks for everyone's efforts.

Do you have a work mail server that you can send via (for testing purposes)?
Can you visit www.whatismyip.com and then copy the IP address into http://www.mxtoolbox.com/blacklists.aspx to see if you are blacklisted.
I just downloaded Vista SP2 (I am also on Ulitmate).  Can you please visit windows update and download anything available and especially any office updates and then try again.

Also - please try telnetting to your SMTP server to see if there is a problem in the flow.  Instructions are below:
http://support.microsoft.com/kb/153119
Please come back with any results.
Alan

When you click on send and receive what is the error which you are getting? can you paste the screenshot of the error?

Also you may refer the article to troubleshoot this issue
http://www.howto-outlook.com/faq/messagestuckinoutbox.htm

I think this may be an issue with Outlook 2007 - I have had issues with it before. Do Comcast provide your ISP services and your Internet connection?

I would try the following:

1. Use any other version of Outlook (if possible) to test.
2. If Comcast provide both services as above, try removing the SMTP authentication so there isn't any (strange I know, but t has worked for me in the past).

Do you have a work mail server that you can send via (for testing purposes)?
No.
Can you visit www.whatismyip.com and then copy the IP address into http://www.mxtoolbox.com/blacklists.aspx to see if you are blacklisted.
98.246.27.138 is b/l'd only in sorbs and spamhaus.
But is this relevant, since Outlook never even tries to send the message (and I can send from other computers with the same settings)?
I just downloaded Vista SP2 (I am also on Ulitmate). Can you please visit windows update and download anything available and especially any office updates and then try again.
The only listing is SP2 for Vista. (I have SP2 for Office.) I will install later today.
Also - please try telnetting to your SMTP server to see if there is a problem in the flow.  
"'telnet' is not recognized as an internal or external command,
operable program or batch file. "
When you click on send and receive what is the error which you are getting? can you paste the screenshot of the error?
No error messages ever.  
Also you may refer the article to troubleshoot this issue
http://www.howto-outlook.com/faq/messagestuckinoutbox.htm
My messages are not stuck. I can easily delete them. They just won't get sent. (I will try creating an empty .pst file when I have some more time.)
1. Use any other version of Outlook (if possible) to test.
2. If Comcast provide both services as above, try removing the SMTP authentication so there isn't any (strange I know, but t has worked for me in the past).
No other version on the problem computer, but I can easily send from any other computer (using Outlook 2007) on my LAN. All the computers have the same mail settings. (I don't use outgoing authentication.)

You would need to install Telnet - Vista does not have it by default.
Please click on Start, Control Panel, Programs nd Features - Left Hand Side - Turn Windows Features on or off.
Choose Telnet Client - wait for ages then it iwll have installed.
Then please try to test again using Telnet.
I will comment on the other findings later - trying to get packed up to go home :-)

Being Blacklisted is not good news - this may also be causing you a problem, but the Telnet should show up something useful.

Can you still send with the other PC's? If so, the blacklisting doesn't seen to be doing anything, although as mentioned above, it's not good.
You mentioned creating an empty PST file on the problem PC - what size is the PST file you are currently using on the problem PC?

I'll install telnet as soon as I get back to my computer, but it seems clear that Outlook is not even trying to send my messages. As soon as I select Send the message appears in non-italics in the outbox.
As I've said, I can send w/o problems from other computers on the same LAN (and can send test messages from the problem computer).
My .pst is 1.6GB. (It's larger than that on the computer I'm now using for Outlook. I copied outlook.pst to that computer several weeks ago, when the first computer stopped being able to send.)

I am still thinking that there is a problem with Anti-Virus / Firewall software - what do you have, can you uninstall it and then re-install it later after testing?
Telnet should highlight something useful I hope.

I'm using the McAfee suite provided by comcast (all current). Disabling it (by preventing it from loading on reboot--I checked that no McAfee services were running) does not change the problem. I'd rather not go through uninstalling and reinstalling it yet.

Sure - totally understand but sometimes it can get in the way even when not running.  I have had cases of other firewall products that were removed, but still retained registry entries that messed up access to the web.
Anyway - go for the telnet route and we can play it by ear from there.

Have you spoken to Comcast at all? If so, have they given you any information?

Could you also post a screen shot of your "Outgoing Server" tab under "More Settings"  please?

I have not contacted Comcast, as it seems clear that Outlook is not even trying to send my messages. They appear immediately in non-italic font in my Outbox (Outlook's way of saying the message will NOT be sent) when I press Send.
Attached are my Outgoing Server and Advanced screenshots.

Advanced.jpg
Outgoing-Server.jpg

I'm sure your other PC's that are working fine with OL2007 are set up exactly the same way. But just to satisfy my curiosity, could you remove the outgoing authentication (clear the username/password and untick "My outgoing server requires authentication) and try again?

I asked about Comcast just to see if they can see any sending attempt (difficult sometimes trying to give answers without "seeing" what's happening). Perhaps it's denying the send very quickly?

Any luck with TELNET?  Do you get anything in your event logs?

No change with clearing and unchecking authentication box.

This was the result of telnet smtp.comcast.net 25:
220 OMTA22.emeryville.ca.mail.comcast.net comcast ESMTP server ready
EHLO comcast.net
250-OMTA22.emeryville.ca.mail.comcast.net hello [98.246.27.138], pleased to meet
you
250-HELP
250-AUTH LOGIN PLAIN CRAM-MD5
250-SIZE 15728640
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-STARTTLS
250 OK
MAIL FROM:robinlake@comcast.net
250 2.1.0 sender ok

Can you continue with the telnet test and put in a rcpt to:<joe@domain.com. (real address) to see if you get rejected please.

I do not believe that this would be productive. Outlook is not even trying to send my email from my outbox, and it is able successfully to send them from Test Account Settings... Obviously, then, whatever happens using telnet is irrelevant to my problem.
To test this belief I disconnected my Ethernet cable and tried to send a message. Send/Receive Progress showed that Sending was successfully completed. (Of course, Receiving reported error, "...Verify that you are online...") And, of course, the message remains in my outbox.
I then tried replacing my outgoing mail server with "xxxx" and again tried to send. Again Sending got its checkmark showing success, and the message remains in my outbox.
I believe that these steps establish that my problem is internal to Outlook, and has nothing to do with my mail server, Internet connection, etc.

Well that is fairly conclusive!
Can you close Outlook down then confirm that it has closed by opening up Task Manager (CTRL + SHIFT + DELETE) - processes tab.
Once closed, please click on start, run outlook.exe /cleanprofile - test sending again
If the above fails, please close outlook again and then try:
start, run outlook.exe /noextensions and test again
then try outlook.exe /safe and test again.
Any improvement?

Have you also run Office Diagnostics?  Help, Office Diagnostics.

Ignore the last request - you have already done this.

No change with /noextensions, but /safe restores the ability to send.

Okay - slight progress
Can you please try with /safe:1, /safe:3 and /safe:4 separately and report back which work and which don't.
Thanks
Alan

For reference - Outlook 2007 command line switches:
http://office.microsoft.com/en-us/outlook/HP012185891033.aspx 

None of the /safe:n permitted sending. Trying /safe by itself again still does work.

Do you get any Application Event Log Errors when not using /safe?
Start, run, eventvwr.exe

The only error is a Warning: "There was a problem reading one or more of your reminders. Some reminders may not appear." Nothing related to sending.

I tried disabling add-ins and found one that was causing the problem. (There were a dozen add-ins, most of which I had no idea were there.)
I will credit alanhardisty with the solution, as he's the one who's spent the most time with me.

Thanks niborr - I am surprised that /safe:3 didn't work as this also disables the addins.
Glad you got the solution and thanks for the points.