Is Email Tracker for Gmail safe?

Email Tracker for Gmail injects a 1x1 tracking pixel into outgoing mail and sends each message's subject and thread ID to its backend.

Running only on Gmail, the extension adds an invisible 1x1 image to emails you send; when the recipient opens the message, that image loads from Mailtrack's servers and records the open. On send it also forwards the message subject line, Gmail thread ID and a per-message tracking ID to the Mailtrack backend, authenticated with your session token. This open-tracking and read-receipt behavior is the extension's stated purpose.