Set-Mailbox -Identity * -ArchiveQuota "Unlimited"

To ensure that the default size of each user's archive mailbox increases automatically when the mailbox approaches the storage limit and to apply the solution automatically to all current and future mailboxes, you can use the Set-Mailbox cmdlet in PowerShell and specify the -ArchiveQuota parameter.

Here is an example of how you could complete the PowerShell command:

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Set-Mailbox -Identity * -ArchiveQuota "Unlimited"

This command will set the archive mailbox size to unlimited for all mailboxes in the tenant.

Please note that, when you set the archive mailbox size to unlimited, the user's mailbox will continue to grow until it reaches the available storage space in the tenant.

You could also set a specific size limit, for example:

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Set-Mailbox -Identity * -ArchiveQuota "50 GB"

This command will set the archive mailbox size to 50 GB for all mailboxes in the tenant.

You can then use the Get-Mailbox cmdlet to verify the new archive mailbox size for all mailboxes in the tenant:

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Get-Mailbox -Identity * | Select DisplayName, ArchiveQuota

This command will return the DisplayName and ArchiveQuota properties for all mailboxes in the tenant.