The two interfaces
An email server stores, receives and sends email on our behalf. Most of us manage our email accounts in one of two ways. Either via a web interface or via an email client. The advantage of accessing our email via a web interface as in Yahoo!, Hotmail, Gmail etc is that there is only one copy of our mail folders (so no problems with copies being out of sync) and mostly we preview everything before downloading.
Low bandwidth access
Because this form of access is so bandwidth friendly, webmail services such as Yahoo!, Hotmail, Gmail etc are able to offer us such accounts for free. Of course, these DO cost money to operate but the expense is defrayed by the fact that these sites gain traction and hence can benefit by offering account holders related products and services. At the very least, the high level of engagement generates a commensurate number of ad impressions.
High bandwidth access
The advantage of accessing our email account via an email client (POP/IMAP access) is that we can read and reply to our mail offline. This can help those who have time-based Internet access plans or those who have their own computer but no Internet access. Because an email client must synchronize mails offline, the process tends to be heavy on the bandwidth requirements and fundamentally inefficient because even unwanted mails and attachments get synchronized.
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