We haven't allowed domain literals (receipt of emails at the IP# address instead of the domain name) for several years.
While it's still in the RFCs, most knowledgeable email admins no longer allow it.
It's useless in a shared domain environment, and will only work for system users only, and not for any domain email accounts, even on dedicated IP domains.
It's been used quite a bit by spammers looking for open relays in the past. While most spammers have abandoned the method, we're still vigilant.
When I wrote the exim.conf file used by DA I made a conscious decision to leave domain literals off by default, but the relevant section of the file is well commented for those who want to turnit on.
Jeff