1. Disable-RemoteMailbox -Identity usersalias (use full email address eg:gerwin@cyp.org.au)
2. Removed the user from the AD sync filter (moved the user to another OU).
3. Forced delta sync.
4. The sync disables the user in azure ad. Went into 365 Admin Center and restored user. Waited until the user's online mailbox reappeared.
5. Adjusted whatever needed to be adjusted (in our case proxy addresses ensure that you use the correct email address eg: SMTP:gerwin@cyp.org.au)
6. Forced Sync again, and reversed steps 2 and 1 – ie Put user back into AD sync filter, Forced Sync again, ran Enable-RemoteMailbox to put the user back into local exchange.
(eg: Enable-RemoteMailbox -PrimarySmtpAddress gerwin@cyp.org.au -RemoteRoutingAddress gerwin@cyporgau.onmicrosoft.com -Identity gerwin@cyp.org.au)