I found the issue located in https://github.com/python/cpython/blob/master/Lib/email/_parseaddr.py#L277

elif self.field[self.pos] in '.@':
    # email address is just an addrspec
    # this isn't very efficient since we start over
    self.pos = oldpos
    self.commentlist = oldcl
    addrspec = self.getaddrspec()
    returnlist = [(SPACE.join(self.commentlist), addrspec)]

The parseaddr function runs a for in loop over the input string, when it meets '.@' it will do something. That is why when the input string is 'foo@bar.com@example.com' will return ('', 'foo@bar.com'). One possible solution will be to check the string in the reverse order then we can always get the last '@' in the string.