A read receipt lets you know when someone to whom you sent an email has opened it. That can be pretty handy, especially when the email bears time-critical information such as travel or meeting plans.
Read receipts are supported in Gmail, but are available only for Google Apps for Work, Education, and Government customers. They are not available in personal Gmail accounts.
There is, however, a workaround using a free email tracking application called Sidekick. Sidekick, by Hubspot, is a sophisticated application that helps you use your mail efficiently alongside your other business systems such as CRM (Customer Relationship Management). I’m not going to get into all of that here, but you can read about it all on their website if it’s relevant to your needs. If you’re using a personal Gmail account it’s less likely you’ll need all the power that Sidekick offers, but you can use it to receive something similar to read receipts.
When someone opens your email, Sidekick pops open a notification window to let you know:
In the free version, mail notifications are limited to 200 per month, but you can decide which messages to track on a per-message basis, so for many people that’ll be plenty. There’s a low-cost premium option for those who need more.
The only slight limitation I’ve found is that, when sending a mail to multiple recipients, you can’t see which recipient has triggered the notification:
Personally, I can live with that.