First, please open the Mail application on the MAC, and then click on the "Mail" menu at the top of the screen, as shown in the following image
Next, click the "Preferences" section in the menu, as shown in the following image
In the Message Settings window, click on the "signature" section at the top, as shown in the following image
Next you can see the mailbox that is configured in your current MAC message, select the message you want to add a signature to, and then click the + button to continue, as shown
Then fill in the window with the signature document you want to set, as shown in the following illustration
Then please write a new message, and the sender's mailbox chooses to create a signed mailbox, and then you see the signature information appearing in the message body, as shown in the figure
By using the same method, you can set multiple message signatures for a single mailbox. To use a different message signature when sending a message, you can select it in the signature Drop-down list. Of course, you can also go directly from here to edit the signature window, as shown in the