Changing the primary SMTP e-mail address in Exchange system is as easy as select a e-mail address & set as reply in E-Mail Addresses tab in Mailbox properties. Well, now it’s greyed out. Baffled? Don’t be.
Answer: Uncheck the option for Automatically update e-mail addresses based on e-mail address policy. That’s it. E-mail address policy is being applied to this mailbox, that’s why you can’t change the default settings.
Anand, the Architect 
thx, it worked
thanks for that! sorted!
Thanks a lot it worked.
I just found this article which solved my problem… thank you for that. Ironically, at the bottom of the article was an add from Staples, the office supply store. In thinking about the answer to this article a Staples marketing phrase comes to mind… “WOW, That was Easy!”. The easy button definitely applies to this solution!!!!!
Ha.ha..Ha. Thanks Chuck.
Thank you it is work
That. . . . was . . . . so simple, yet so elegant. lol I feel kinda dumb now, but thanks man it worked.
Years later, this article is still helping people like myself- thank you!
Thank you saved me hours of research and troubleshooting!! Mahalo
Thank noble man
Tks 🙂
Doesn’t work
After you disable “Automatically update email addresses…” check box, click OK to save the settings. Reopen the mailbox properties, then you can change the default email address.
Anand