Try this assuming you are using Outlook 2003. I'm not sure about Outlook 2007:
Find out if your inlaws ISP using dynamic or Static IP address. If it's dynamic then goto "My Network Places", right click and select Properties. Select your LAN connection and then Properties. Go to Internet protocol TCP/IP, make sure "Obtain IP address automatically" and "Obtain DNS Server automatically" is selected. If it uses a static IP address make sure you get all the IP addresses, gateway and DNS server information and enter manually. OK out of these windows. Reboot the computer is asked.
1) Open Outlook, goto Tools, Select "Email Accounts"
2) Goto "View or change email account".
3) Keep the POP3 server the same, but change the outgoing email server to the the inlaws ISP outgoing server, using the exact name that is required.
4) Goto "More Settings" and select "Outgoing Server" tab.
5) Check the box that says: "My Outgoing server requires authentication" if your in-laws ISP requires it. If not, leave it unchecked.
6) Important Step
Select: "Log on using" Enter your IN-LAWS email address under username and your IN-LAWS password for their email account. Also check the "remember password" box. Do not use your own here, since you need to instruct Comcast to send a message, not your current ISP.
7) Now go the "Advanced" tab. Find out what the Comcast outgoing Server port number is and change it to that number. Leave the incoming as is.
Click on any apply buttons and then OK out of each dialog box. Close Outlook and wait about 10 seconds.
Re-open outlook and try to send a message.
Let me know.