You can have up to 6 additional email accounts in Comcast besides your primary account. Sign on to your primary account and click on the Users & Settings tab at the top of the page. You should see a link in blue underneath your user name that says Create Secondary User.
If you notice yellow leaves in other areas of the tomato plant, or the yellowing at the bottom is spreading upward, this could be a sign of disease, such as curly top virus (usually curly top virus will show signs of yellowing leaves that are curling up as well), Ringtop Virus, or other diseases.
The most likely culprit, I think, is going to be item (3) above. At least, that is what the problem is in my large tomato garden 95% of the time when leaves begin yellowing and/or curling. If you have bacterical speck, you will notice tiny specks of brown to black to almost purple on the leave surfaces.