Hi,
Mailspring has a paid feature to get read-receipts each time an email is opened.
I guess it uses web beacons, right? Web beacon - Wikipedia
But is it still possible to request for a free return receipt, like in other MUA like Thunderbird? Return receipt - Wikipedia
Mailspring’s read receipts work via a tracking pixel (web beacon) rather than the older IMAP Return-Receipt-To / Disposition-Notification-To header approach. The IMAP-based return receipts require the recipient’s mail client to actively send back a response, which most modern clients don’t do automatically. The tracking pixel approach is more reliable in practice. Both are Pro features in Mailspring. You can read more about how it works here.