The mxcache.dat file is located in \MDaemon\app. Open the file with Notepad to edit it. Keep a copy as a backup.
The commented text in the mxcache.dat file has instructions and samples:
# Use this file if you want to cache MX hosts for certain domains.
#
# Keep in mind that all the entries in this file are stored in RAM
# at all times for performance reasons. As a result if you edit
# this file you must either restart MDaemon or create a semaphore
# file called MXCACHE.SEM in the \App\ directory so MDaemon will
# reload any changes.
#
# Lines beginning with # are comments. Create entries of the form:
#
# Domain Name Precedence Host [{IP Address}] [AUTH Logon] [AUTH Password].
o365-v4.vaultastic.com 10 {domainprefix}.vaultastic.com