How to convert secondary email address (alias) in Office 365 to shared mailbox using Powershell

I know there's a 30 day recycle bin when you delete a user's mailbox. The situation I have is this: user was originally set up with a mailbox using companyA.com as UPN and primary mailbox. Not realizing the use case for their name in another domain would require configuring it as a shared mailbox instead of an alias, the user got a secondary mailbox with companyB.com domain. After trying to delete the secondary address and creating a shared mailbox with the same name, it was obvious that the 30 day recycle bin effect was happening.

Before the knee jerk answer of "delete the mailbox and start over" - this is the president of the company and they already have mail activity in the newly migrated Office 365 environment, so we're NOT deleting the whole user.

I've found the samples online of where people delete user mailboxes from the recycle bin using PowerShell. What I'm looking for, since I'm very new at PS, is the language, if it exists, to accomplish the same thing with the secondary addresses of people who actually need that secondary address converted to a shared mailbox. I know there's a convert command for a mailbox which would convert the user's primary mailbox (not just the address), but I'm hoping to either convert the additional email address or delete the additional email address, remove it from the recycle bin immediately, and then create a shared mailbox without the 30 day waiting period.

I tried renaming the secondary once also, but I still got the error message about trying to create a shared mailbox that had a "proxy address already being used" . Any ideas how to accomplish this?
Shannon MollenhauerAsked:
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Shannon MollenhauerAuthor Commented:
Another curiosity which may explain why this is such a problem:

I tried creating a shared mailbox with a different name for companyB.com and when I looked at the email addresses for this mailbox, it also created a secondary for companyA.com (my default domain). I tried to delete the address but when I hit save, it still appeared in the addresses for the mailbox.

Is this going to be a problem for everything where I might have a user mailbox in my default domain and a shared mailbox in another domain trying to use the same "user" prefix? Any way to avoid this other than changing my default domain back to the onmicrosoft.com domain?
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Vasil Michev (MVP)Commented:
But you already have 2 user accounts for him, as you cannot have more than 1 mailbox associated with the same user object. So you will be pretty safe to delete it.

Anyway, best thing to do is to confirm to which object the alias is currently attached. This can easily be done via the following cmdlet:

Get-Recipient user@domain.com

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After you locate the object, you can decide what to do with it. Removing only the alias is the safest option.
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Shannon MollenhauerAuthor Commented:
Perhaps I need to clarify the account situation. We have multiple domains under the same Office 365 tenant structure, but the domains are technically two separate entities, just managed as sister companies, so upper management have accounts in both domains. They need to be able to send email using the second company domain addresses and signatures so we should not have created aliases with the second domain names but rather just shared mailboxes.

But, this problem of the shared mailboxes for the second domains also getting automatic assignments with a secondary address using the default domain makes me think the whole process is hosed.

The get-recipient command returned the same name for both domains, as I would expect, since I put the secondary alias back on for now so the president gets email from that company as well until I figure this out. It reported RecipientType as UserMailbox for both, too.

Is script that will remove the extra address assignment and also remove it from the recycle bin right away so the secondary domain's shared mailbox can be created?
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Vasil Michev (MVP)Commented:
While you can create a shared mailbox with a different primary smtp address, it will indeed have an alias associated with the default domain added as well. You can remove that after creation or in bulk later via PowerShell. You do not need to delete the recipient object. Once the alias is removed, you can immediately assign it to another existing object, or create a new one.
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Shannon MollenhauerAuthor Commented:
Each time I have tried to delete the alias associated with the default domain it re-appeared in the admin interface. Do you have a sample PS command I can try?
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Vasil Michev (MVP)Commented:
Try this:

Set-Mailbox shared -EmailAddresses @{Remove="shared@domain.com"}

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Shannon MollenhauerAuthor Commented:
I used a new-mailbox command to create a shared mailbox and used Vasil's set-mailbox code to remove the proxy addresses from the users and the arbitrary shared primary domain configuration. PowerShell is appropriately named - Powerful but a tough shell to crack! Thankfully, PS can do things the web interface can't or won't let you.
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