User unable to login and the portal doesn't recognise his email username

[Annalisa Afeltra]

I have a user that once and a while has problems logging into the portal and cannot do a reset password cause the service desk portal doesn't recognise his email address.

I haven't requested if he has plugins installed but wanted to know if there is any reason this could happen?

Attached is the screenshot of the error. We did verify that he had no spaces in the email.