According to Wikipedia (https://en.wikipedia.org/wiki/Email_address) and http://isemail.info/about the maximal length of the local part of an email address is 64 characters.
However, I just received email from this address: reply+0032ff332e028331fad75f7549ee52d90483c7aa70138a3192cf00000001123b88e492a169ce06aab82c@reply.github.com
Its local part is 90 characters and it is deemed invalid by isemail.info, however, it's a perfectly valid email address. I can send email to it and it is received by the other party.
So what gives: is not the maximal length of the local part of email address 64 characters or not? If not, what is the maximal length then?
Yet as MSalters says in comments, just because something is done doesn't mean it's legal.
Some system accept longer local parts, some others don't. In this case, Github says that you should send an e-mail to them on that address. It's bad practice from Github because they might accept a longer e-mail address, but they forget that the client might be more pedantic and refuse to send (or worse, truncate the e-mail address).
They probably consider reply as the real local part and use +0032ff33... as an identifier, but all in all, as you point out, it makes their local part much (too?) bigger.