Originally Posted by JSanders
I don't know how much data is used... I am sure that might vary from email to email, however, your BB only downloads the headers of the message, not entire emails and/or attachments. So, as you stated, if you do not open the message and delete it, you are using the least amount of data for that message.
I am sure the first 1,500 bytes are downloaded at the same time as headers. To confirm this I did a quick test and sent an email to my BB with about 7k of text. When it arrived I opened the message and watched the top right corner for data flow and also held it close to my PC speaker which buzz when the BB is acknowledging received data. When I opened the message on the BB there was no data flow. (I use a GSM based 8700, CDMA users may not detect the data noise issue)
I then used the SELECT mode and copied all of the data in the BB email up until the device said "Select MORE for an additional 5,600 bytes" (balance of the message) and sent it to my home account. Here I did a character count that stated there were 213 words and 1,211 characters not including blank spaces. I summed the two for the estimate there are about 1500 bytes sent.
Bottom line in my case is that my Data usage excluding the headers, would be a maximum of 1,500 bytes per message.
Now a very neat feature of the BB BIS/BES is when you reply to a message. In this sample case I quoted one may expect they would be billed 1500 bytes PLUS their response. Not so... if your reply was simply "OK", you would only be billed for 2 bytes plus any header charges. The BES prepends your response to the original message and delivers it just as if it had been sent from the device.
Ask your PALM friends how their data usage burns through carrier caps