+If
+.I fetchmail
+is used with a POP or an IMAP server, it has two fundamental modes of
+operation for each user account from which it retrieves mail:
+\fIsingledrop\fR- and \fImultidrop\fR-mode. In singledrop-mode,
+.I fetchmail
+assumes that all messages in the user's account are intended for a single
+recipient. An individual mail message will not be inspected for recipient
+information, rather, the identity of the recipient will either default to
+the local user currently executing \fIfetchmail\fR,
+or else will need to be explicitly specified in the configuration file.
+Singledrop-mode is used when the fetchmailrc configuration contains at
+most a single local user specification for a given server account.
+.PP
+With multidrop-mode,
+.I fetchmail
+is not able to assume that there is only a single recipient, but rather
+that the mail server account actually contains mail intended for any
+number of different recipients. Therefore,
+.I fetchmail
+must attempt to deduce the proper "envelope recipient" from the mail
+headers of each message. In this mode of operation
+.I fetchmail
+almost resembles an MTA, however it is important to note that neither
+the POP nor IMAP protocols were intended for use in this fashion, and
+hence envelope information is not directly available. Instead,
+.I fetchmail
+must resort to a process of informed guess-work in an attempt to
+discover the true envelope recipient of a message. Even if this
+information is present in the headers, the process can
+be error-prone and is dependent upon the specific mail server used
+for mail retrieval. Multidrop-mode is used when more than one local
+user is specified for a particular server account in the configuration
+file. Note that the forgoing discussion of singledrop- and
+multidrop-modes does not apply to the ESMTP ETRN or ODMR retrieval
+methods, since they are based upon the SMTP protocol which
+specifically provides the envelope recipient to \fIfetchmail\fR.
+.PP