This is a flow of mail through the internet from a sender to a receiver; depicted is a fully-fleshed flow, though some steps are optional. The simple picture is: sender - transfer - recipient: MUA - MTA - ... - MTA - MUA, but there can be finer details.

action	actor	pass to next actor by	step
			Sender wants to send a message.
write new email	MUA (mutt itself, and mutt-var <tt>$editor</tt>)	\/ mutt-var $sendmail \/
queue & send	local MTA (optional)	\/ SMTP \/
validate & accept	ISP's MSA	\/ SMTP \/
			now email is stored at own ISP's MSA = ISP's MTA
relay to next MTA	internet MTA	\/ SMTP \/	this repeats for each MTA between source & destination
			now email arrives at remote ISP
receive	remote MTA	mta-internal
deliver	MDA	\/ rules, filters, forwards \/
store	ISP filesystem	delivery complete
			Recipient now wants to read the stored mail.
action	actor	get from previous actor by	Note: The activity direction changes. So far the sender "pushed" the msg, now the receiver has to "pull" the msg.
access & store locally	MRA (optional)	/\ IMAP, POP /\
read	MUA	/\ local filesystem /\
			Success: the message has gone from sender to recipient.\

Recipient may want to become the sender of a different message; start over.

Several steps above are optional:

• The local MTA is omitted on most desktop computers; instead, the MUA directly submits the message to the ISP's MSA.
• The MSA/MTA distinction is relatively recent, and the ISP may not distinguish between user submissions and message transfers.
• The MRA step is included as part of most desktop MUAs, though traditionally email was read directly from a local or shared (as by NFS) file, and still is if one uses a shell account and mail spool on the server to read mail. A separate MRA such as fetchmail is used to replicate the traditional "MUA doesn't handle delivery" design, and allows very light-weight mail checking; in integrated MUAs it may be a separate thread or process.