Workaround for Office 365 SharePoint Site Mailbox App mailbox name rewrite issue

Site Mailbox is an App available in SharePoint Online  that helps by providing a central place to file email and documents that can only be accessed and edited by those with the appropriate site permissions.

When creating a Site Mailbox in SharePoint Online, a client and I ran into a funny anomaly. We know that there is a limitation in that once you’ve created a site mailbox, you can’t change its email address. Since the email address for a site mailbox is the display name of the site, when you name the site, you normally choose a name that will also work well as the email address. That’s all fine.

The expectation we had in my client’s scenario was to use with the 3 digit project number + Project Title.  An example of the desired address we wanted would be:

..the bolded text being the SharePoint Site Title “193 My Project Name” with its spaces removed by the system.

The important part for the client was the project number. Problem is, if one makes a site title with just a number, or leading with a number, Office 365 inexplicitly strips it out, and replaces it with seemingly random characters:

SharePoint Site Title Used: “193” Generated Site Mailbox Address as created by SharePoint Online:

As explained, if we just go with their current Site Title, the leading digits (and space + dash + space) get removed- no good. There is no way to change the Site Mailbox address after creation (ref: It’s not an option in either the GUI or via PowerShell.

My Workaround

In my scenario, the Client already had an internal established notation to prefix projects with the letter “p”. Therefore it would acceptable to prefix the project number with a letter.

1. For each existing or new SharePoint site you intend to add a new Site Mailbox App, temporarily alter the site name (Site Settings > Title & Description) to begin with a single letter. In this case, we altered all their existing Project sites in the format (note the “p” prefixed on the site title):

Site Title: “p193 My Project Name

2. Deploy the Site Mailbox App. The email address generated will come out as in the style

3. Once Site Mailbox App is fully deployed, change the Title of the site in Site Settings back to “197 My Project Name” (without the “p”), therefore avoiding inconsistencies elsewhere including in document tags, metadata & last but not least, leaving people wondering why there’s a wacky letter at the start of your site title. J 4. That’s it, you’ve somewhat hacked your way around this strange little behaviour of how email addresses are generated when creating a Site Mailbox. Conclusion As for why the SharePoint Online system really does not seem to like email addresses that start with numbers, I have no info at this time. Certainly an address of would meet general email RFC specs. Why the address comes out as SMO + alphabet soup + is a mystery and seemingly undocumented. I suspect it would be part of a secret sauce that only a select few MS Engineers may know – or it’s a simple pattern that’s obvious from the output email addresses. If anyone knows the reasoning behind the email address rewriting in play with site mailboxe, please do share in the comments!


5 responses to “Workaround for Office 365 SharePoint Site Mailbox App mailbox name rewrite issue”

    • Matthew, note that in the URL for that article it has “/en-in/” meaning English-India. I think what might have happened is either:

      – That article across all language variants was generally incorrect in saying that one could rename the site mailbox address, and they updated them since you posted this comment, or,
      – The main en-us version got updated to reflect correctly that you can’t change the specific email address of the site mailbox, but, for probably logistical/editorial reasons the English-Indian version was behind the times and eventually got updated.

      In any case, the documentation matches the reality now: there’s currently no mechanism to change the site mailbox email address.

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