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Email Spam Filtering
Scanning of all emails for spam is an optional module available for NetManager. This page assumes you have already purchased this and that it is installed.
All emails travelling through the system (either in or out) are assessed by the spam filter. Mail that has been received from trusted networks (e.g. your LAN) will be passed through without further checking. All other emails are rated according to a number of different metrics. For example:
- How much it looks like emails that are known to be spam (Bayesian filtering)
- Sending IP address is on known blacklists
- Mentions Viagra or other such pharmaceuticals
- From: address is incorrectly formed
- Message is HTML only
- Date is clearly wrong
- Mail format is incorrect
The full list can be seen here.
Each mail will be given a numeric rating and it is this rating that determines what happens to it. Firstly, we should distinguish between clearly spam and suspected spam.
Mails with a very high rating (by default 10) are almost certainly spam and should be rejected or dropped. Mails with very low ratings are safe and should be delivered. Intermediate mails are probably spam, but we can't be 100% sure (after all, one person's junk mail is another person's must-have subscription). Therefore mails above a certain lower number (by default 5) will be flagged as spam (***SPAM*** will be added to the Subject line), but they will still delivered. The mail will be altered so that it shows which particular tests it failed.
Default behaviour.
- Mail with a rating of 10 or higher will be rejected. This will send the mail back to the sender. However, as most spams use made-up addresses (or addresses of innocent victims), this can result in backscatter whereby innocent people receive bounced emails that they did not send.
- Mail with a rating of 5 or higher will be flagged as spam, but still delivered to the original user
- All other mail will be delivered as usual
Other options
- Do not reject any mail (to avoid back scatter), but:
- Deliver all mail flagged as spam to another mailbox or:
- Drop all mail flagged as spam silently (but beware of false positives).
N.B. Currently the software only allows you to redirect all mail flagged as spam, not just that above an upper limit (i.e. you can't just redirect definite spam whilst continuing to only flag suspicious mail). You must be wary of false positives (i.e. interfering with good mails).
Cnfiguration options
The spam filter can be altered with the following configuration options in the configuration file:
spam_level (default 5) - level above which mails are flagged as spam
spam_reject (default 10) - level above which mails are rejected. Set to 0 to not reject any mails
spam_bayes (default y) - whether to enable Bayesian filtering
spam_autolearn (default y) - whether to mark senders who have sent mail rated spam before even more highly
spam_redirect - email address to redirect mail above spam_level to. Set to devnull to silently drop
