And you can send to this e-mail from your other computer with using the same e-mail account to send from?
If that is the case, look and see if you have this person in your contacts and if so remove the contact from outlook and try sending a e-mail to him. If that works you can go and add him back to the contacts. Some times a outlook contact can go bad. Or try sending to this contact after putting your computer in safe mode with networking ( if you are able to send to him in safe mode it's your anti virus that is doing this). If these don't work create a new profile in outlook ( to do this you have to go to the control panel click on the mail icon and then click on show profiles from here you will see ADD create a new profile and try send the e-mail. Make sure you enter all the right info for you e-mail account whe setup the new profile.) AFter creating the new profile you will see a option to prompt for a profile to be used ( make sure you put a dot in this oprion)


But if you are sending to this person from another computer, but using a diff e-mail account the problem is with your sever. ( most servers have a option on them to be able to block e-mails and you may have this e-mail blocked for sending to without even knowing it) As a microsoft outlook tech this is something I see often.

Also who is your e-mail account with ? and your ISP?