How get custom property

I am developing a custom Routing Agent for Exchange server. I was able to reach MailItem.Properties["Microsoft.Exchange.Transport.Directory Data.Sender.RecipientLimits"], but it doesn't suit me. It was decided to create a custom property for the purposes of the agent's task. I need to get a sender mailbox custom property on which my agent's logic depends. In the admin panel, it is obtained as follow "Get-Mailbox | fl customattribute1" How do I get this property in one of the transport agent events? I am currently using the OnResolvedMessage event.

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