Creating New Exchange Mailbox results in extra characters on user name and undeliverable mail.

I've attempted to create a new user account and Exchange keeps messing up the username.

I'm using Exchange 2010. Every time I attempt to create a new mailbox -- whether through Exchange Management Console or creating a user account on the PDC then create the mailbox for an existing user -- I end up with a variation of that user account with some extra characters added to the end.

For example:

I've attempted to create "construction@mycompany.com" and then when I send an email I get an undeliverable message with the following error:

#550 5.2.0 resolver.pf.invalid; misconfigured public folder mailbox ##

And upon inspecting the account I find that the default SMTP reply addresses looks something like

construction2f7a3@mycompany.com
construction2f7a3@mycompany.local

And every time I try to remove the extra characters under Email Address I get an error:

"The command will clear the PrimarySMTPAddress property because the EmailAddressPolicyEnabled parameter is set to true. This action is not allowed."

(so it won't allow me to make the change.)

I tried making a distribution group and the same thing happened.

This is Exchange 2010 running on Windows Server 2008 R2 (yes, I'm already planning the upgrades and migrations, awaiting approval on new backup software first).

On a sidenote, the PDC on the network is a Windows Server 2016 and a few months ago we had to use it to seize the FSMO roles from an SBS 2003 server. I mention this because I'm wondering if there could be some residual issues left over from SBS that is causing this problem.

Any insight as to what could be causing this and how to fix it would be greatly appreciated.

Thanks in advance!