When an email address contains invalid characters (i.e. on-ASCII) mailmerge crashes with a stack trace.
Expected behavior: some kind of error message instead of crash. Ideally, continuing to send the remaining emails.

Example: email: mü[email protected] which contains an umlaut that shouldn't be there.
Result: UnicodeEncodeError: 'ascii' codec can't encode character '\xfc' in position 10: ordinal not in range(128)