I've created retention tag which deletes mail after 6 month after tag was applied. Users are using this retention tag in their mailboxes for new mail messages. I need to know if it’s possible to do that retention tag (Office 365 label) which is applied for new mail message will be valid not only in sender mailbox but in receiver’s mailbox too.
No. The tag is only valid for and can be applied to items in mailboxes within your organization. If you are looking to protect messages sent externally, try the Azure Information Protection/Office Message Encryption services.