Password change mail still send

On my site the mails for the password change of a user are still sent.
When I try the same on a test site with only a few plugins active your plugin works fine.
How can I identify the potential problem with another plugin?

If I display the hook list for wp-mail I see
>>>>> wp_mail
10 wp_staticize_emoji_for_email
alo_em_handle_email

I think the last function is yours?
Shouldn’t that have a priority number?
I can see pluggable-functions-1.3.php gets activated.
I tried the suggestion of another post of changing false to __return_false but that did not help.

I use WP 5.0.2