3. Compressing Email
3.1 Incoming Emails (POP3, IMAP4)
Most people fetch their email from the mailserver via POP3. POP3 is a protocol with many disadvantages:
- POP3 transfers password in clear text. (There are SSL-implementations of POP/IMAP and a challenge/response authentication, defined in RFC-2095/2195).
- POP3 causes much protocol overhead: first the client requests a message than the server sends the message. After that the client requests the transferred article to be deleted. The server confirms the deletion. After that the server is ready for the next transaction. So 4 transactions are needed for each email.
- POP3 transfers the mails without compression although email is highly compressible (factor=3.5).
You could compress POP3 by forwarding localhost:110 through a compressed connection to your ISP's POP3-socket. After that you have to tell your mail client to connect to localhost:110 in order to download mail. That secures and speeds up the connection -- but the download time still suffers from the POP3-inherent protocol overhead.
It makes sense to substitute POP3 by a more efficient protocol. The idea is to download the entire mailbox at once without generating protocol overhead. Furthermore it makes sense to compress the connections. The appropriate tool which offers both features is SCP. You can download your mail-file like this:
scp -C -l loginId:/var/spool/mail/loginid /tmp/newmail
But there is a problem: what happens if a new email arrives at the server during the download of your mailbox? The new mail would be lost. Therefore it makes more sense to use the following commands:
ssh -l loginid mailserver -f mv /var/spool/mail/loginid /tmp/loginid_fetchme
scp -C -l loginid:/tmp/my_new_mail /tmp/loginid_fetchme
A move (mv) is a elementary operation, so you won't get into truble if
you receive new mail during the execution of the comands. But if the mail server
directories /tmp/ and /var/spool/mail are not on the same disc you might get
problems. A solution is to create a lockfile on the server before you execute
the mv: touch /var/spool/mail/loginid.lock. You should remove it, after that.
A better solution is to move the file loginid in the same directory:
ssh -l loginid mailserver -f mv /var/spool/mail/loginid /var/spool/mail/loginid_fetchme
After that you can use formail instead of procmail in order to filter /tmp/newmail
into the right folder(s): formail -s procmail < /tmp/newmail
3.2 Outgoing Email (SMTP)
You send email over compresses and encrypted SSH-connections, in order to:
- Save network traffic
- Secure the connection (This does not make sense, if the mail is transported over untrusted networks, later.)
- Authenticate the sender. Many mail servers deny mail relaying in order to prevent abuse. If you send an email over an SSH-connection, the remote mail server (i.e. sendmail or MS-exchange) thinks to be connected, locally.
If you have SSH-access on the mail server, you need the following command:
ssh -C -l loginid mailserver -L2525:mailserver:25
If you don't have SSH-access on the mail server but to a server that is allowed to use your mail server as relay, the command is:
ssh -C -l loginid other_server -L2525:mailserver:25
After that you can configure your mail client (or mail server: see "smarthost") to send out mails to localhost port 2525.
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