There are 5 types of forwarding in MS Exchange Online. Two are properties of the mailbox and 3 are types of rules. Note: This script can take a long time to run in large organizations.

• MSOnline PowerShell module

  Install-Module -Name MSOnline -Scope AllUsers

• ExchangeOnlineManagement PowerShell module

  Install-Module -Name ExchangeOnlineManagement -Scope AllUsers

• Make folder c:\temp if it doesn't exist
• Check if the MSOnline and ExchangeOnlineManagement modules are installed
• Get all of the mailboxes in your tenant
• Get every domain that is a part of your tenant (acceptedDomains)
• For every mailbox:
  • Check to see if it has a forwarding property (DeliverToMailboxAndForward, ForwardingSmtpAddress, ForwardingAddress)
  • If it does AND is going to a domain outside of your tenant accepted domains THEN export to CSV
  • Get every rule
    • For every rule:
      • See if it is a forwarding rule (ForwardTo , or ForwardAsAttachmentTo, or RedirectTo)
      • If does AND is going to a domain outside of your tenant accepted domains THEN export to CSV
• Results will be saved to the following CSV file formatted with date and time the script was run:

c:\temp\exolForwarding_DATE_TIME.csv

• Once finished running, the script will tell you:
  • The location of the output
  • The time the script started
  • The time the script finished

• PrimarySMTPAddress of the mailbox
• DisplayName of the mailbox
• Rule priority (for a rule)
• Rule description (for a rule)
• Rule ID (for a rule)
• Rule enabled status (for a rule)
• Rule name (for a rule)
• Rule ForwardTo address (for a rule, if exists)
• Rule RedirectTo address (for a rule, if exists)
• Rule ForwardAsAttachmentTo address (for a rule, if exists)
• Mailbox DeliverToMailboxAndForward status (mailbox property, if exists)
• Mailbox ForwardingSmtpAddress (mailbox property, if exists)
• Mailbox ForwardingAddress (mailbox property, if exists)