Link User to Existed Exchange Online Mailbox

Powershell Exchange Online

Get-Mailbox UserName | fl ExchangeGuid    #取得 ExchangeGuid

PowerShell On-Premises

Enable-RemoteMailbox UserName -RemoteRoutingAddress  UserName@YourDomain.mail.onmicrosoft.com
Set-RemoteMailbox UserName -ExchangeGuid <ExchangeGuid from above>

如果是 Shared Mailbox, Enable-RemoteMailbox 指令加上 -Shared 參數

參考資料:
https://www.itpromentor.com/link-aduser-365/