The Mail Merge for Gmail app lets you include an unsubscribe link in your email message. If a user clicks the unsubscribe link to opt-out of your mailing lists, you’ll not be able send future email campaigns to that email address.
You can always send emails to the unsubscribed user manually via Gmail but they will not be included in the email campaigns sent via Mail Merge for Gmail.
If your contact has unsubscribed their email address from your mailing list accidentally, or if you manually unsubscribed a user from your email system, you can easily reset their subscriber status from withing the Mail merge app. You’ll then be able to resume sending email campaigns to the resubscribed user via Mail Merge.
Open your Google Sheet, go to the addons menu, select Mail Merge with Attachments, choose Email Campaign Reports and then choose Resubscribe Contacts.
On the next screen, type one or more email addresses that you wish to reactivate for Mail Merge. You can specify up to 4 different email addresses, and each email address should be specified in its own input box.
Click the Resubscribe button to reactivate the subcriber.