Changed sender email field max length

It should be 254 (http://stackoverflow.com/questions/386294/what-is-the-maximum-length-of-a-valid-email-address), but that seems improbable.. so I would go with 50 chars max, instead of 20 that cuts 4 out of 10 emails I receive.
Merge pull request #858 from nicoschtein/patch-10

Changed sender email field max length