Monica is the only one that fits the description.
I will admit, she does fit the description.....kind of.
You do realize that the mystery e-mailer cannot be Monica. The whole purpose of the e-mail, was to send MONICA the MC's phone number, and info.... and inform her that the MC's father was dead.
Thereby starting the beginning of the game, Monica herself had a whole kitchen table conversation on the matter, as she swears that the MC e-mailed her and was going to show him.....even though he swears that he didn't send it even under the extreme duress of losing his most beloved father.
Pretty sure she did not get the MC's info, through Madam's magic palm reading, then go to a coffee shop to use their computer and then e-mail herself.
Just so she can finally have the MC's info and call him, inviting him to spend time with her, after the love of his life is finally planted six feet under.
Plus you've got
CheekyGimp who is not going to make it obvious. He had made a point of misdirect, by having the MC's thought dialog make reference to her name. Plus a few others that it could Not possibly be too.
Peace!!