If you don't know of Exchange (2010) E-mail Address policies, you should. If all your company's people should have an address firstname.lastname@example.org and a secondary address email@example.com and then suddenly a new address firstname.lastname@example.org -- you need E-Mail Address Policies.
Not only that, but you also need PowerShell, since the Exchange console Email Address Policy editor does not support arbitrary email address templates. One address template it can't handle is just that, firstname@anywhere, for which you need Powershell.
Fire up your Exchange Management Shell and enter something along this line:
Set-EmailAddressPolicy -Identity 'Default Policy' -EnabledPrimaryAddressTemplate "smtp:%email@example.com" -EnabledEmailAddressTemplates "smtp:%firstname.lastname@example.org", "smtp:%email@example.com"
That last bit was the magic one. Substitute the 'Default Policy' to whatever policy you're editing and the somedomain.com to suite your organization.