Resolve issues with shared mailboxes
If you see error messages when creating or using a shared mailbox, try these possible solutions.
Error when creating shared mailboxes
If you see the error message, The proxy address "smtp:<shared mailbox name>" is already being used by the proxy addresses or LegacyExchangeDN of "<name>". Please choose another proxy address, it means you're trying to give the shared mailbox a name that's already in use. For example, let's say you want shared mailboxes named info@domain1 and info@domain2. There are two ways to do this:
Use Exchange Online PowerShell. See this blog post for instructions: Create Shared Mailboxes with Same Alias at Different Domains
Name the second shared mailbox something different from the start to get around the error. Then in the admin center, rename the shared mailbox to what you want it to be.
Error about not having send permissions when using a shared mailbox
If you created a shared mailbox and then try to send a message from it, you might get this:
This message could not be sent. You do not have the permission to send the message on behalf of the specified user.
This message appears when Microsoft 365 is experiencing a replication latency issue. It should go away in an hour or so, when the information about your new shared mailbox (or added user) is replicated across all of our data centers. Wait an hour and then try again to send a message.
About shared mailboxes (article)
Create a shared mailbox (article)
Configure a shared mailbox (article)
Convert a user mailbox to a shared mailbox (article)
Remove a license from a shared mailbox (article)