There is a setting under the QQ email name at the top of the page, as shown by the arrow in the figure below. Click Settings.

A personalized signature was found under the general page.

4. Click Add Personalized Signature, enter the signature name, enter the signature content and confirm. You can save multiple signatures.

Click Edit to open the editing window, where you can modify the signature and confirm the save.

5. At the bottom of the letter editing and sending page, select the signature you need at the position of the red arrow. The black arrow indicates the effect.