External E-Mail tagging in Exchange Online
We all know the e-mail disclamer and “EXTERNAL” tags in subject lines that should make clear, that a specific e-mail is coming from external sender. Mostly this is done to make sure nobody clicks on links in external e-mails that might look like an external e-mail.
This can easily be done by creating a transport rule in Exchange or Exchange Online that matches senders outside the organization. This rule adds something to the beginning of the subject line, and usually a preamble is added to the mail body.
