It could also help to move the button away from the search input in the UI and/or to change the label to "Clear all messages".

Are you mainly wanting to prevent accidental usage?

The clear button does ask for confirmation with a pretty clear message:

You will lose all your received messages.

Are you sure you want to clear all messages?

Yes, but nevertheless it happens from time to time in our setup.

Therefore, in my opinion, a clearer message on the button and/or another button location would be helpful. I think the main problem is that material design (and others) suggests/uses to use a "clear this input" trailing icon on the right side of the input itself and that people are used to this pattern...

I have an additional perspective to @rustysponge.

I believe its natural behaviour for a user when he has a task requiring him to use mailcatcher, that he clears all emails to have a fresh start with less clutter. That is cool in a single user environment.

In a multi user environment this user might delete all mails to declutter his experience, but having done this, might have deleted mails other users spent significant time on preparing scenarios for. Rather than solving this through processes (weekly memo: please everyone, do not clear the mailcatcher, think about the needs of your colleagues) a functionality preventing the user from doing so would solve that automatically.

Therefore from my perspective there are two parts. One is unintentional use of the clear functionality and the other is unintentional side effects of using the clear button.

For our org something like this would do the trick:

mailcatcher --messages-limit 500 --no-clear

Hello,

We had exactly this same issue this morning: someone thought that the button was about clearing the search field. The person then clicked quickly the confirmation message.

Afterward, the person said that the confirmation message was indeed clear.

It seems that like other say in this issue, the "Clear" button could be moved or modified to be more precise.

I've seen that the linked pull request is no more in sync, how would you to move on this issue?
Could I propose a PR with a rewording or a change in location of the button ( or search box)?
It could be as easy as taking the same text for the HTML title and content of button, see code

Regards,