Changing the default email address in Office 365

We ran into a problem last week where account managers noticed that the reply-to address on the emails that they were sending was defaulting to onmicrosoft.com. I assumed that this would be an easy fix, so I logged into the Office 365 admin and tried to change the default email address to the @sherpamarketing.ca email. I was presented with the following error message:

After trying many different changes, I stumbled upon a post that walked me throught the following steps:

1. Run as ADSIEDIT.msc as an Administrator

2. Add entries to the mail and proxyAddress values:

3. Force a sync.

This seems to have corrected the problem for us.