Here are a few suggestions for optional modifications you might want to make to the shared-mailbox solution. Note that if you've changed which extensionAttribute to use, you have to change the text for the selected attribute if you make any of the following changes.

Modify GAL Search Filter
When you’ve created a shared mailbox, both the mailbox and mail-enabled security group will by default be visible in the Global Address List (GAL) in Outlook or Outlook Web Access (OWA). For more information about how to find or modify the GAL search filter (the Active Directory—AD—purportedSearch attribute), see the Microsoft article “Newly created mailbox or distribution list is not displayed in the Global Address List” (http://support.microsoft.com/?kbid=315531).
(Be careful: There’s an error in the article, which Microsoft hasn’t yet corrected. The following string is missing from the GAL search filter that the article shows: objectCategory=msExchDynamicDistributionList. Be sure to add this string to the filter, if you plan to use dynamic distribution lists.

To avoid confusing users, you shouldn’t show shared-mailbox security groups in the GAL. Users could easily choose the wrong receiver when two objects are similar. Typically they should choose the shared mailbox and not the group. You can modify the GAL search filter to avoid confusion by inserting the string (!(extensionAttribute1=SMowner)) into the purportedSearch attribute containing the existing GAL search string. Then the shared-mailbox security groups won’t appear in the GAL. The search string should then look like somthing like this.

Modified GAL search filter

 (&(!(extensionAttribute1=SMowner))(mailnickname=*)(|(&(objectCategory=person)(objectClass=user)
(!(homeMDB=*))(!(msExchHomeServerName=*)))(&(objectCategory=person)(objectClass=user)
(|(homeMDB=*)(msExchHomeServerName=*)))(&(objectCategory=person)(objectClass=contact))
(objectCategory=group)(objectCategory=publicFolder)(objectCategory=msExchDynamicDistributionList)))

Hint: When you want to modify the GAL search string by using the ADSI Edit tool, I advise you to try your string before you save it (see the previously mentioned Microsoft article for more information about what could happen when you incorrectly set the filter). An easy way to see the effects of your changes to the string is to find the objects and count them by doing a GAL preview in Exchange System Manager (ESM). Start creating a new address list, do a custom search, insert the string you want to try, and press Preview to see the objects and new count. Compare the results and see whether the difference is as expected. Then use ADSI Edit to copy and paste your string into the purportedSearch AD attribute and cancel creation of the new address list.

Address Lists for SM Objects
Exchange administrators can also set up additional address lists to separate the different objects. You can also use such lists to narrow search results and also speed up a user’s search to confirm email addresses.To do so, create two new address lists called Shared mailboxes and Shared-mailbox owners. The following are examples of custom search strings you could use for these address lists:

• Shared mailboxes: (extensionAttribute1=SM)
• SMowners security group: (extensionAttribute1=SMowner)

The address lists are useful for finding shared-mailbox and shared-mailbox–group objects and viewing naming conventions when you’re planning to create a new shared mailbox.