Are you arguing in bad faith, or do you just not have any practical experience dealing with complex systems?
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.
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.