Are comments allowed in email address domain part?

actually, comments are allowed, whereas I have to add that - generally speaking - the comment itself should be either at the end or at the beginning of each part:

(comment)[email protected]
local.part(comment)@domain.part.tld
local.part@(comment)domain.part.tld 
[email protected](comment)

I think, if there is a comment inbetween it should be surrounded by ' " ' + dots:

local."(comment)"[email protected]

as special characters like "(" must be in ' "'.

There is a great summary on wikipedia with references. https://en.wikipedia.org/wiki/Email_address

Whereas for me, the questions is left whether the comment can span the whole local part.

Crazy, but it's legal, from RFC2822 (from chapter: A.5. White space, comments, and other oddities):

From: Pete(A wonderful \) chap) <pete(his account)@silly.test(his host)>

..The above example is aesthetically displeasing, but perfectly legal...