I’ll tell you what you can mail.
You can mail letters.        
You can mail a toy train.
You can mail a toy train by rail—by a real train.
You can mail a toy plane.
You can mail a toy plane by a real plane—by airmail
You can mail ginger ale, a sail, and a nail. 
You can mail a pail.
But you can’t mail a snail in a pail.
You can’t mail a whale.
And you can’t mail a real plane or train!
The mail carrier puts a toy train and a toy plane on the scale and mails them off.