Since GMass uses the native Gmail Compose window as the basis for campaign creation, and since GMass allows you to copy/paste a list of addresses right into the To field, I thought it would be fun to see just how many email addresses the To field of the Compose window can hold. Is there a limit? Is there a connection between how many addresses Gmail will let you put in the Compose window and Gmail’s actual email sending limits? Would the Gmail anti-spam pre-crime team detect that I’ve pasted thousands of email addresses into the To field and shut down my account before I even have the chance to hit Send (or the GMass button)?

Let’s find out.

1. First, I need a big list of email addresses I can paste in. Easy enough — I use SQL Server to store my users’ data, so I can just copy/paste email addresses from SQL Server into the Gmail Compose window.
2. Here I’ve pasted 1,980 email addresses in the To field. Note that in the screenshots below, the Gmail Send button is hidden by GMass, to prevent a user from making the grave mistake of hitting the wrong button.
   
3. Might as well keep going…6000 email addresses!
   
4. As the number of addresses gets bigger, each subsequent paste into the To field gets slower, and additionally, I get an increasing number of Chrome “Page Unresponsive” alerts.
5. A whopping 15,000 email addresses.
   
6. From 15,000 I went to 20,000 with ease. I then decided to jump from 20,000 to 50,000. After clicking “Wait” on no fewer than 20 of the “Unresponsive Page” dialogs, and after waiting about 45 minutes, finally, the Compose window revived itself to life, with almost 50,000 addresses.
   

How do I know if the Compose box is still working?

As a Gmail expert, I’m used to the behavior of elements of the Compose window, so I would regularly test these to make sure they are still behaving normally. For example, I know if that if I type a few words in the Message area, the URL will change to reflect a new Gmail Message ID, since Gmail has just re-saved the Draft. I know that I should be able to click the X next to an email address in the To field to remove it. The count of total addresses should be accurate.

Copy/paste addresses directly into the To field, or use Google Sheets?

Even though I’ve shown that the To field can hold plenty of email addresses, it’s still more efficient to paste a large number of email addresses into a Google Sheets spreadsheet rather than into the Compose window directly. Why?  Because when you put your addresses into a spreadsheet, when GMass connects to it, it creates one single “alias” email address that represents all of the addresses in the spreadsheet. So the Compose window To field will only show one address. When the To field shows thousands of addresses, the JavaScript required to process all of that email address data slows down the inner workings of the Compose window significantly. Additionally, with the CPU resources Chrome will consume just to handle that Compose window, your entire computer is likely to slow down during a large copy/paste operation into the To field. Here is my CPU usage right now, as I attempt to go from 20,000 addresses to 50,000 addresses (so pasting 30,000 more) into the To field.

In Conclusion

While in theory it seems that the Gmail Compose window can hold an unlimited number of email addresses, it becomes impractical to paste so many addresses after the 10-20,000 address mark. The Compose window reacts too slowly, you get too many “unresponsive” dialog boxes, and the waiting time required for the “paste” operation to complete is unbearable. If you’re using GMass to send mail merge campaigns with Gmail, then I recommend using our Google Sheets integration if you’re needing to send to more than 20,000 addresses.

Ajay Goel

Ajay is the founder of GMass and has been developing email sending software for 20 years.