If you select an address book, then you must be certain that you want an email to go out to everyone in that address book. It may be best to create a new address which is only used for the Mail Merge. You can edit the address book to add or remove contacts as required. Please note, selecting an address book means every contact written in that selected address book must have a valid email address in the 'Email' field. Else it will fail.

This is also true with a .csv file. All column headers must be identical to the ones you use in the email template. To use {{Email}} in template email, the column header in .csv file must be exactly the same: Email

I would recommend that you use the 'Send Later' option as this will put all the individual emails into the 'Outbox' so you can check them.

See example images as guide.

I was able to get the csv version working after I realized that Excel SaveAs CSV does not delimit the text. It only puts in the commas. It seems that most examples on the Web use cvs files.

As for the Address Book I was careful to select a test address book so the drafts would be limited. I gave up on getting to work with email or Email or PreferredEamil.

I'm also having issues after further testing with address bok option. I noticed that info on the addon webpage mentions a known bug with ldap. Perhaps this is the issue. Exporting address book and using as .csv file works perfectly ok.