Error 550 is an authenticaion error. I assume that you're using an ISP to send mail. Try these things...

I'm using Outlook 2000, so I'm telling you what the settings are for me... Outlook has an option to send mail from a differnet server and authenticate you separately. Find the Outgoing Mail Server option (should be under Mail Options and find the server information (where your email server name(s) are set up) Where this section is depends upon whether Outlook is configured as an Internet email or Corporate/Workgroup... The incoming and outgoing email server names may be different.

In this section, check the "My server requires authentication" checkbox. Click the "Settings" button and select "Log on Using" part and enter your login information there (usually your ISP username and password). Click OK, OK, OK, OK, Apply or whatever and try again!

Other than that, pray... a lot! Good luck!