Are you receiving the following error when emailing users within the same domain name?

Diagnostic information for administrators: Generating server: EXMBX01.infoquest.com

IMCEAEX-_O=HOSTEDEXCHANGE_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=mailbox@uniqtest2007dom.com
Remote Server returned ‘550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found’

Within Outlook, Click the File Tab > Click Options >

Click Mail then scroll down and click the button labeled “Empty Auto-Complete List”.

This will resolve the issue.