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Added mail agent figure.
author markus schnalke <meillo@marmaro.de>
date Wed, 11 Jul 2012 15:51:28 +0200
parents 4c5055a0e981
children 05a243dffaca
comparison
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186:5d6dca258633 187:5360f5fdb118
7 is described in the examples of how the general goals were achieved. 7 is described in the examples of how the general goals were achieved.
8 The discussion compares the current version of mmh with the state of 8 The discussion compares the current version of mmh with the state of
9 nmh just before the mmh project started, i.e. fall 2011. 9 nmh just before the mmh project started, i.e. fall 2011.
10 Current changes of nmh will be mentioned only as side notes. 10 Current changes of nmh will be mentioned only as side notes.
11 .\" XXX where do I discuss the parallel development of nmh? 11 .\" XXX where do I discuss the parallel development of nmh?
12 .P
13 For the reader's convenience, the structure of modern email systems
14 is depicted in the figure.
15 It illustrates the path a message takes from sender to recipient.
16 .sp
17 .KS
18 .in 2c
19 .so input/mail-agents.pic
20 .KE
21 .sp
22 .LP
23 The ellipses denote mail agents, i.e. different jobs in email processing:
24 .IP "Mail User Agent (MUA)
25 The only program the user interacts directly with.
26 It includes functions to compose new mail, display received mail,
27 and to manage the mail storage.
28 Also called \fImail client\fP.
29 .IP "Mail Submission Agent (MSA)
30 A special kind of Mail Transfer Agent, used to submit mail into the
31 mail transport system.
32 .IP "Mail Transfer Agent (MTA)
33 A node in the mail transport system.
34 Transfers incoming mail to a transport node nearer to the final destination.
35 It may be the final destination itself.
36 .IP "Mail Delivery Agent (MDA)
37 Delivers mail by storing it onto disk, usually according to a set of rules.
38 .IP "Mail Retrieval Agent (MRA)
39 Initiates the transfer of mail from a remote server to the local machine.
40 (The dashed arrow represents the pull request.)
41 .P
42 The dashed boxes represent groups that usually reside on single machines.
43 The box on the lower left represents the sender's local system.
44 The box on the upper left represents the first mail transfer node.
45 The box on the upper right represents the transfer node responsible for the
46 destination address.
47 The box on the lower right represents the recipient's local system.
48 Often, the boxes above the dotted line are servers on the Internet.
49 Many mail clients, including nmh, have all of the components below
50 the dotted line implemented.
51 Not so in mmh, which is an MUA only.
52
53
54
12 55
13 56
14 57
15 .\" -------------------------------------------------------------- 58 .\" --------------------------------------------------------------
16 .H1 "Streamlining 59 .H1 "Streamlining
80 This was a departure of the dominant through of the time, 123 This was a departure of the dominant through of the time,
81 in which routing logic, local delivery, and often the network code 124 in which routing logic, local delivery, and often the network code
82 were incorporated directly into the user agents. 125 were incorporated directly into the user agents.
83 .QE 126 .QE
84 .P 127 .P
85 In nmh, the Mail Submission Agent (MSA) is called 128 In nmh, the MSA is called \fIMessage Transfer Service\fP (MTS).
86 \fIMessage Transfer Service\fP (MTS).
87 This facility, implemented by the 129 This facility, implemented by the
88 .Pn post 130 .Pn post
89 command, established network connections and spoke SMTP to submit 131 command, established network connections and spoke SMTP to submit
90 messages to be relayed to the outside world. 132 messages to be relayed to the outside world.
91 The changes in email demanded changes in this part of nmh as well. 133 The changes in email demanded changes in this part of nmh as well.
117 This appears to be the better interface. 159 This appears to be the better interface.
118 .\" XXX implement it 160 .\" XXX implement it
119 .P 161 .P
120 To retrieve mail, the 162 To retrieve mail, the
121 .Pn inc 163 .Pn inc
122 command acted as an Mail Retrieval Agent (MRA). 164 command acted as an MRA.
123 It established network connections 165 It established network connections
124 and spoke POP3 to retrieve mail from remote servers. 166 and spoke POP3 to retrieve mail from remote servers.
125 As with mail submission, the network connections required encryption and 167 As with mail submission, the network connections required encryption and
126 authentication, thus TLS and SASL were added. 168 authentication, thus TLS and SASL were added.
127 Support for message retrieval through IMAP will soon become necessary 169 Support for message retrieval through IMAP will soon become necessary