i tell you the procedure:
user write an email to a office 365 group that forward to an external contact on hotmail where osticket check a specific folder if there are new mail, and when "ticketized" mail will be moved on another folder on hotmail.
so user will reply to the mail coming from hotmail. in fact on "failed" alerts, i cannot see any "sent items" from hotmail.
i don't know, but, do you think that could be a potential issue that my "user mail" registered in osticket is the same admin mail registered in osticket? and this can cause issues also to other users?