Depending on your Preview setting for mail - the number of preview lines shown for a received message dictates how much is downloaded initially with each new message, but another preview line for text does not amount to much data. When you open a received message, the remainder of the message is downloaded. Depending on the size of an included attachment, the attachment may or may not be downloaded when opening the message. If an included attachment is not downloaded when opening the message, you must select the attachment to download the attachment from the server.
Depending on your Preview setting for mail - the number of preview lines shown for a received message dictates how much is downloaded initially with each new message, but another preview line for text does not amount to much data. When you open a received message, the remainder of the message is downloaded. Depending on the size of an included attachment, the attachment may or may not be downloaded when opening the message. If an included attachment is not downloaded when opening the message, you must select the attachment to download the attachment from the server.
I don't know what your data plan limit is, but having "preview 5 lines" for received messages alone without opening any received messages and without knowing how many messages you receive on a monthly basis should not cause you to exceed your data plan limit especially if you don't access any other app that requires internet access when not connected to the internet via an available wi-fi whatever your data plan limit happens to be.
The phone uses data to download the e-mail. It's not using any when you're reading it. Depending on your settings and how much e-mail you receive is going to influence the amount of data used. Then again most e-mail doesn't use a large amount of data in the first place.
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When I'm reading email, am I using data from my data plan?
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