One nice feature when using IMAP within mail.app is the ability to list multiple addresses in the “email address” field separated by commas. This allows me to have both my real email address as well as any aliases defined. For instance, I might have firstname.lastname@example.org as well as email@example.com separated by a comma. With this configured, if someone sends something to firstname.lastname@example.org, when I hit "reply-all" my alias is not listed in the To: or Cc: fields.
My company recently upgraded to Exchange 2010, and I've converted to using the "Exchange" account type to gain the additional integrations as well as calendaring. However, the problem is that now I no longer have the option to change or add additional addresses to the “email address” field in the Mail settings for the Exchange account. (It’s grayed out) Has anyone found a workaround for this issue?
Exchange sucks in this and, IMO, many other ways. At my companies' exchange server I have my "main" email address, plus about 10 aliases. I can receive mail from any of the aliases. But when I had my Exchange account configured as an IMAP server in Mail (even on SL and earlier) any time I sent, regardless of the from address I specify in my client, it is always converted on the Exchange server to be from my "main" address. It works exactly as you describe for all of my other account types, just not Exchange.