Do you have an <input> tag in the modify form for that field? If so, what type of tag is it? Can you post the tag so we can see? If you do not have an <input> tag in your modify form, and you don't get the error with the add a link form that users submit, then something else is wrong someplace. Check the following things in links.def:
1. Your ReceiveMail field definintion in %db_def looks like this:
Quote:
ReceiveMail => [13, 'alpha', 10, 10, 1, 'Yes', 'No|Yes']
2. %add_system_fields contains an entry like this:
Quote:
ReceiveMail => 'Yes'
3. %db_select_fields contains:
Quote:
ReceiveMail => 'Yes,No'
1 thru 3 above is based on the assumption that the field is required and not changeable via the add/modify forms by the user.
I hope this helps.