Imagine you're writing trading software, you have an algo go haywire and it machine guns the whole order book, and then you refuse to put a "max order size" outside of the algo to stop it from happening again because you can't figure out why it happened in the first place.
Try telling a regulator or your boss that was your reasoning.
Even if the bug can't be reproduced, on the basis of multiple user reports the first step absolutely should be to add some assertions and logging around email deletion.
The point is not to give it a "100% safe" label, the point is to start narrowing down possible root causes. If the problem recurs again, you'll have assertions ruling out certain possible culprit code paths as well as logs displaying the values of relevant variables.