The person who emailed you is trying to tell you
Harry Potter and the Cursed Child is bad.
Your mind wanders back to the effort they put into hiding this message...
Clicking on the image you found a picture of a bookcase and a computer. The bookcase contained links which led to other images. A lot were filler images, but there were a few that caught your eye...
Some of the images seemed to contain hidden text. The life of Fred had Q3 and h2 quite obviously in it. Harry Potter and the Deathly Hallows contained Y1.
The trickiest to find
Was in The Hobbit. It is very hard to see but there is a q5 in the top right above the brim and a p4 in the top left corner. You had to zoom in a lot.
Ordering them from their numbers gave
http://i.stack.imgur.com/YhQpq.png gives us
The numbers seeemed to correspond with
The numbers in the email. To decode you must take the number in the email, find its match and replace it with the letters
By decoding the entire email with this you got
Hello, Mithrandir. Once you and your little friends have decoded this, go to UOSvt.png. I will wait and watch.
So you went to
To solve it you had to go back to the bookcase and to the book labelled Saruman. In it you found
A hidden imgur link - https://i.stack.imgur.com/R7xgk.png
Which gave you
The small text at the bottom was
Bayl jbeqf gung ner 3 yrggref ybat - ROT13 for
'Only words that are 3 letters long'
So you solved the crossword with words 3 letters or longer and were found only a few letters remained unused.
The letters left over were:
You also had to add an 'i' from the hidden link in the 'divergent' link:
And an 's' from the Gas Gage.:
So the final letters were:
Which descrambled to:
Cursed Child: Bad.
You sccessfully unlocked the email's secrets.