@Paris Geek has already answered your question.
In addition to that, - an idea of how you can lower your data usage even further:
1. Do not use a high number for "Messages to sync".
2. Decrease frequency of scheduled sync.
3. Under Settings -> Network -> check the settings at the bottom, under "Mobile Data Settings" section, especially the 2nd and the 3rd in that category. This is especially helpful if you receive many messages that you don't need to read right away (or at all). In that case, - only a very small portion of a message (1 kB or headers only) is downloaded on mobile network. Not that when you are reading the message, the remainder of it will be downloaded if you scroll down.
Hope this helps.