Comment by speak_plainly

8 hours ago

Probably Handoff and cross-device application state syncing.

Most likely answer so far. While writing an email you can take it over between iPhone, iPad and Mac. Once you hit send it probably gives a “you can stop Handoff now” signal.

I've disabled Handoff.

Also, why would Handoff be triggered by sending an email rather than composing an email? Mail app uses IMAP for syncing mailboxes.