Yet Another Mail Merge

(One time only) Install the free add-on if you haven't done so already.

PREPARE THE DATA

Go into Drive and create a new blank Google Sheets spreadsheet.

Go to the Add-ons menu and select Yet Another Mail Merge -> Start Mail Merge.

It will prompt you about recipients. Click "Add emails myself."

It'll build a sample layout:

Leave the "Email Address" and "Merge status" headers alone. You can change/add any other columns (paste from Excel). E.g.:

CREATE THE LETTER TEMPLATE

Now, switch to another tab/window and go to Gmail and click Compose. Create the letter wording, formatting, etc. and use <<FIELDNAME>> where you want your spreadsheet's dynamic data to land. Leave the "To" field blank and save the draft (it will autosave as you go.) E.g.:

RUN THE MERGE

Now, return to your spreadsheet and go back to the Add-ons menu and select "Start Mail Merge" again. From the "Email Template" dropdown, choose your draft, and for "Sender Name" type whatever you'd like to appear to recipients.

THEN, click "Alias, filters, personalized attachments..."

...and enter the email address you want replies to go to:

Then click "Back", then "Send emails." Let it churn for a few minutes. It should then update the status in the Merge status column with "EMAIL SENT."

TRACK RESULTS

Recipients will get an email like this:

Note that, even though it was sent from the "crossroads" account, if they click Reply, it will go to Peter.

When a recipient opens a message, it will change to "EMAIL OPENED":

Any bounces will be noted as well.