Q: How should the email address field be completed?
A: For item 19 – Email Address:
Enter the person's e-mail address if known. Include all formatting, punctuation, and special characters in the e-mail address. An e-mail address must contain the “@” sign with a period in the following text, e.g. email@example.com or firstname.lastname@example.org. Leave Item 19 blank if the e-mail address is unknown.
This is addressed in the "FinCEN Currency Transaction Report Electronic Filing Requirements" guide, Attachment C.