Iphone SMS character limit different when sending via email

Hello,

I have found that when sending a text message via email to mynumber@txt.att.net that if you put the message just in the subject line it cuts it off at 40 characters and you don't get multiple text messages. If you put the message in the body of the email it cuts off at approximately 120 characters, but it will break it up into multiple text messages. Is this just a limitation when sending texts to my phone via email? Is there part of the email header or something that is being counted that I don't see? I tried this same text using a verizon phone and it sends 160 regardless of whether it's in the subject or body.