User Agent Types
There are two types of user agents: command-driven and GUI-based. Command-driven
user agents belong to the early days of electronic mail. They are still present as the
underlying user agents in servers. A command-driven user agent normally accepts a onecharacter
command from the keyboard to perform its task. For example, a user can type
the character r, at the command prompt, to reply to the sender of the message, or type
the character R to reply to the sender and all recipients.