Make sure your using the most up to date version of Handcent (I believe it is 2.9.21) because I know they were doing alot of work on MMS capabilities.

Also, I was having general MMS "issues" when I first picked up the Droid but I found that by updating all of my mobile numbers in my address book to the 1-312-555-1212 format my messaging issues were solved. I would take out the parentheses on the contact you want to message, like the format I described above, reset the phone and then try sending the photo again. If it works, update all of your contacts.

I updated mine by exporting a google.csv file from my gmail contacts. I did a "file - replace" using excel and then reimported my google contacts into gmail which then updated my Droid OTA. I reset the phone and Voila!

 

To reset: Shift-ALT-Delete or just turn it off, pull the battery for a second or two just to be sure and power up again.

 

...and you've got to remove the parentheses too, not just add the 1

 

I ran some tests this morning and at least to my AT&T iPhone friends the MMS messages are getting thru but they do seem to be slow. Maybe 10 minutes or so later the message might pop up.

 

Gmail Contact Formatter || 11/18/2010 - xda-developers