Regulation 12(4)(e) states:
12.—(4) For the purposes of paragraph (1)(a), a public authority may refuse to disclose information to the extent that—
(e) the request involves the disclosure of internal communications.
Regulation 12(8) states:
12.—(8) For the purposes of paragraph (4)(e), internal communications includes communications between government departments.
As with all EIR exceptions, this is a qualified exception. It may be straightforward to determine whether the requested information is internal communications, which engages the exception, but you must then apply the public interest test set out in regulation 12(1)(b). You can only withhold the information if the public interest in maintaining the exception outweighs the public interest in disclosing the information.
Regulation 12(2) states that you must apply a presumption in favour of disclosure.