Based on your description,
if I sent an email from our domain to an external user you can find the signature attached at the bottom of the email and when the external user reply the signature will be there. How come that our signature is attached to external user?
I would like to confirm that what you want to ask is why the signature is appended to the external user's reply, or is it some other issue? Please correct me if I understand it incorrectly.
In this case, the signature will actually be included as part of the message content. When an external user replies to an email, their email client automatically includes the entire email thread in their reply, including the signature. Therefore, its presence in the reply email is the expected behavior.
Additionally, it is recommended that you refer to this section of the documentation to add an exception that prevents multiple disclaimers from being added in an email conversation.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.