How to undo Send

“You can stop a message from being sent. Front will hold your message for a selected amount of time before sending it. You can enable the undo send option on every type of channel. By default, undo send is enabled on email channels and disabled on all other channels (Twitter, SMS, etc).”

Here’s the Front article on how to enable and configure this option:
How to undo send (front.com) 

Once you ‘Undo’ the sending of a message, it can be edited and re-sent. 

Note:  Internal comments relating to a particular message/thread can also be Edited (pencil) / Deleted (trash can) (See Example Below)