Gregory's Server@dansup I have also seen success with asking users to go to their inbox and search for the mail domain to find it - assuming the pixelfed welcome/confirm mail content includes the username :)
"""
"Forgot your email?
No worries! You can search your inbox for "from:pixelfed.dk" and check your welcome mail - otherwise, put your username in the form below, and we will send you an email"
"""
@jippi Some people don't remember which account they used though, this saves admins a bit of time by automating the experience in an ideal way.