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## General overview of the mail processing chain
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A generic (yet quite common) mail processing chain.
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```
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------------ managed by ----------------
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| mua |<-----------------------------------------------| user mailbox |
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|(e.g. mutt) | |(e.g. ~/Maildir)|
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------------ ----------------
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| ^ ^
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| compose the message and send | |
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| it through a pipe | |
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v | | -------------------
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------------- | | | external filter |
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| SMTP client | | | |(e.g. spamassassin)|
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------------- | | -------------------
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| | | | ^
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| | | | |
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| ------------------- file transport | | v |
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`---------> | SMTP local server |------------------------- ---------------------------------
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,---------> | (e.g. exim) | pipe transport | mail delivery/processing agent |
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| | |----------------------------->| (e.g. procmail) |
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| | |------- ---------------------------------
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| ------------------- |
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------------------------ |
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| pop retrieval program | | SMTP transport
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| (e.g. fetchmail) | |
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------------------------ |
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^ |
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| v
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------------------- --------------------
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| remote pop/imap | | remote SMTP server |
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| server | | |
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------------------- --------------------
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``` |