Edit the default recipient policy to change email addresses

So your user account is first initial lastname but you want the email address to be just the first name, ok.

Open ESM- Recipients-Recipient Policies and edit the default policy. Then under E-Mail Addresses (policy) edit the SMTP address to be:
%g@domain.com for it to be firstname@domain.com
%g.%s@domain.com for firstname.lastname@domain.com
%1g%s@domain.com for first initiallastname@domain.com
You can create multiple addresses at the same time as well by clicking new.