Add IMAPFilter

diff --git a/linux/misc.md b/linux/misc.md
index 74a67ffc914563221ccd9eeb7cc8a9b95c0b04ac..326a5ad54830cb9fe1b0059c0c153e7a37d27190 100644 (file)
--- a/linux/misc.md
+++ b/linux/misc.md
@@ -37,3 +37,41 @@ sudo smem -P beam.smp -kta
 `ssh -D 1234 host` creates a Socks proxy on `localhost:1234` that sends traffic through `host`.
 
 By enabling "allow extension to control proxy settings" in the [multi account containers](https://addons.mozilla.org/en-US/firefox/addon/multi-account-containers/) Firefox add-on, you can make containers use specific proxies.
+
+## Email forwarding via IMAP
+
+When you have multiple email addresses, you have multiple options to use them.
+
+Traditionally, people used redirection/forwarding to make email arriving at `from@example.com` go to `to@example.net` instead.
+
+If mail from `example.com` and `example.net` is handled by different servers, typically you can configure the `example.com` mail server to resend any message arriving to the `from` address to `to@example.net`.
+
+However, nowadays with spam filtering, the `example.net` mail server can reject these emails as spam, sometimes silently.
+
+For safer redirects, you can:
+
+* Use the same mail server for both accounts.
+  However, this sometimes cannot be done or has extra cost and complexity.
+
+* Configure the destination email server to fetch email from the origin mail server.
+  For example, Gmail can do this, but the fetching period can be as long as 15 minutes.
+  This can be inconvenient when receiving confirmation emails, etc.
+
+  Additionally, operators of the destination email server now have your credentials.
+
+A third option is to run this fetching process yourself.
+
+<https://github.com/lefcha/imapfilter> supports connecting to an IMAP account, waiting until messages to arrive, and moving them to another IMAP account.
+
+Benefits:
+
+* IMAPFilter can use IMAP idle to request the IMAP server to notify when messages arriving, so forwarding happens without a polling delay.
+* Because IMAP is used on both sides, no spam filtering happens.
+* IMAPFilter is [packaged for many distributions](https://repology.org/project/imapfilter/versions).
+
+Drawbacks:
+
+* Requires additional infrastructure.
+* If IMAPFilter stops working, email stops being forwarded without warning.
+
+Refer to [this Python module](../scripts/p7s/mail/__init__.py) for scripts that configure IMAPFilter as a systemd service, with credentials from Bitwarden.