Data Masking Email Using Formulae

Hi guys, 

I am trying to work on my data masking skills and have ran into a problem. I am trying to mask the email addresses of the following data set (this is a fake set of data from an online course btw).

 I want to mask the emails such that: email address removed for privacy reasons will then become b******email address removed for privacy reasons . 

I want to make sure that the number of Asterisks represents the number of characters displaced, I have spent a while trying different formulae, however I have been able to generate a formula for the first email from E2 into F2, although this doesn't seem to work for the other rows of emails.

Any help is much appreciated :)