You must add a signature to the selected email once you set the sender's signature under the Sender menu.

To add a signature click the Personalize menu above the email text editor and select SENDER SIGNATURE which looks like {{SENDER_SIGNATURE}}

 

Under Sender info, you can also find other Sender-related variables, that you can include anywhere in the email copy.

You need to add {{SENDER_SIGNATURE}} to every email where you want it to appear. This way of adding Sender-related variables gives you the flexibility to mix email structure. For example, the initial email has a signature, while the follow-up only has the sender's name.