<P> Most modern graphic email clients allow the use of either plain text or HTML for the message body at the option of the user . HTML email messages often include an automatically generated plain text copy as well, for compatibility reasons . Advantages of HTML include the ability to include in - line links and images, set apart previous messages in block quotes, wrap naturally on any display, use emphasis such as underlines and italics, and change font styles . Disadvantages include the increased size of the email, privacy concerns about web bugs, abuse of HTML email as a vector for phishing attacks and the spread of malicious software . </P> <P> Some web - based mailing lists recommend that all posts be made in plain - text, with 72 or 80 characters per line for all the above reasons, but also because they have a significant number of readers using text - based email clients such as Mutt . Some Microsoft email clients allow rich formatting using their proprietary Rich Text Format (RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client . </P> <P> Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents (MTAs); and delivered to a mail store by programs called mail delivery agents (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem . </P> <P> Users can retrieve their messages from servers using standard protocols such as POP or IMAP, or, as is more likely in a large corporate environment, with a proprietary protocol specific to Novell Groupwise, Lotus Notes or Microsoft Exchange Servers . Programs used by users for retrieving, reading, and managing email are called mail user agents (MUAs). </P>

All email services are based on which software model