Thank you for your post.
You can import the bad email addresses into another table, and have a relationship linked by email address. Then, in your main contact database, you can have a portal into the bad email addresses, and perform a count in a calculation field. That is:
Count ( BadEmails::Email )
That is, assuming the table is named BadEmails, this would count the entries of any emails that are bad. You could then search for >0 in the calculation field, or just search for 0 in this field for valid addresses. Does this make sense?
Your solution does make sense to me - will let you know the results! Thanks so much for your response - makes me feel much more confident about using Filemaker as I struggled linking tables properly but finally am getting it. I may be posting some additional issues as I move forward.