Sending reported error(0x800CCC78)

The server responded:550 206.91.xxx.xxx is blacklisted.

That error is quite clear. Have you verified this or contacted the ISP who owns that mail server already?

It would not explain why you would not get this error when using another mail application unless you have configured it for another mail server.

You can try recreating your mail profile to make sure no corruption in it is the cause of it;
http://www.howto-outlook.com/faq/newprofile.htm

Do you have a virus scanner installed which integrates itself with Outlook? Uninstall this integration part of your virus scanner and try again; you'd still be sufficiently protected by your on-access scanner part of the virus scanner. For more details see;
http://www.msoutlook.info/question/20

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Robert Sparnaaij [MVP-Outlook]
Outlook guides and more: HowTo-Outlook.com
Outlook Quick Tips: MSOutlook.info

That error is quite clear. Have you verified this or contacted the ISP who owns that mail server already? It would not explain why you would not get this error hen using another mail application unless you have configured it for another mail server.

As I mentioned, please don't fall for that error! Did I not mention that other mail clients like thunderbird is working fine for the same account on the same server?

Also, I'm able to telnet to the same smtp server and use auth plain to login and send a mail from the same server using cmd!

That error is coming up only because outlook is not performing the logon before sending an email by sending the AUTH PLAIN LOGIN message which the other mail client is doing just fine. Please refer to the part about how I found this out with the packet analyzer

You can try recreating your mail profile to make sure no corruption in it is the cause of it;
http://www.howto-outlook.com/faq/newprofile.htm

Do you have a virus scanner installed which integrates itself with Outlook? Uninstall this integration part of your virus scanner and try again; you'd still be sufficiently protected by your on-access scanner part of the virus scanner. For more details see;
http://www.msoutlook.info/question/20

As mentioned, other pop accounts are working fine for smtp authentication in outlook itself, so, I don't see a reason why even if the virus scanner was integrated, it would be blocking a particular account and not others on the same email client. Also, no virus scanner is integrated.

Recreating the profile is the last thing I want to do at this point considering the pain in restoring all the accounts and emails back on. To test the profile theory, I went back into Outlook express and created a new profile and setup the account there and it's the same problem. As a note, I had long back migrated this account from OE to Outlook2k3, so I suspect it's the same account cache issue!

I had the same problem and found something that worked for me.  I went to tools>email accounts>select the account>more settings>outgoing server>check the box that says "my outgoing server requires authentication>select "use same settings as my incoming mail server">hit ok and then test the settings

Again, this worked for me so maybe it will for you as well.  Let me know.

Thanks!

I had the same problem and found something that worked for me.  I went to tools>email accounts>select the account>more settings>outgoing server>check the box that says "my outgoing server requires authentication>select "use same settings as my incoming mail server">hit ok and then test the settings

Again, this worked for me so maybe it will for you as well.  Let me know.

Thanks!

OMG....Thank you sooooo much!!! Your solution worked perfectly.  I've been trying for weeks and not one person could find the answer. 

I had the same problem and found something that worked for me.  I went to tools>email accounts>select the account>more settings>outgoing server>check the box that says "my outgoing server requires authentication>select "use same settings as my incoming mail server">hit ok and then test the settings

Again, this worked for me so maybe it will for you as well.  Let me know.

Thanks!

Thank you for your suggestion, but with all due respect, that is something that I tried first as I've been working with outlook for several years. In most cases, this would work, but not in mine.

I had the same problem and found something that worked for me.  I went to tools>email accounts>select the account>more settings>outgoing server>check the box that says "my outgoing server requires authentication>select "use same settings as my incoming mail server">hit ok and then test the settings

Again, this worked for me so maybe it will for you as well.  Let me know.

Thanks!

OMG....Thank you sooooo much!!! Your solution worked perfectly.  I've been trying for weeks and not one person could find the answer. 

Just came across the same problem and Cajun Frog's solution was excellent, thanks very much

I have the same issue with my Comcast mail. In the Tools => view account info I need to change the setting => server type to authorization required (check box in upper right). If I go back to the account info tab and perform the Test everything including the send passes. If I go back the Outlook and try to send a message I get the same error. When I go back to the Account info settings page everthing is back at the default values.

I have the same issue and i followed CajunFrog solution and it worked for me.

Worked for me too - thanks CajunFrog

Fantastic, this worked! I tired registryfix, then a system restore, nothing worked. I tried to see if outlook's own repair system would do the job, but your suggestion worked immediately!