diff --git a/NixOS.md b/NixOS.md
index dfd1768..a0dd5f0 100644
--- a/NixOS.md
+++ b/NixOS.md
@@ -238,6 +238,23 @@ nix-eval '{ a=1; }?a'
 #=> true
 ```
 
+## Escape strings
+
+* https://nixos.org/manual/nix/stable/language/values
+
+Double-quoted strings (e.g. `"foo bar"`):
+```
+"  -> \"
+\  -> \\
+${ -> \${
+```
+
+Indented strings (e.g. `''foo bar''`):
+* `${` -> `''${`
+* `''` -> `'''`
+
+URIs can be written without quotes (e.g. `http://example.org/foo.bar`).
+
 ## Run AppImages
 
 * https://nixos.wiki/wiki/Appimage
@@ -249,4 +266,15 @@ nix-shell -p appimage-run
 ```shell
 # Inside the shell, you can run an AppImage:
 appimage-run ~/Downloads/ubports-installer_0.10.0_linux_x86_64.AppImage
-```
\ No newline at end of file
+```
+
+## Additional resources
+
+* Simple NixOS flake template for NixOS with HomeManager. https://github.com/Misterio77/nix-starter-configs#readme
+* NixOS guide (development environment, gaming, etc). https://github.com/mikeroyal/NixOS-Guide#table-of-contents
+* nix function cheatsheet: https://nix-cheatsheet.pixie.homes/
+* Documenting the Journey of Learning and Mastering Nix. https://ianthehenry.com/posts/how-to-learn-nix/
+* Nix language basics. https://nix.dev/tutorials/nix-language.html
+* Nix data types. https://nixos.org/manual/nix/stable/language/values
+* MkIfElse. https://discourse.nixos.org/t/mkif-vs-if-then/28521/4?u=langfingaz
+* Nix language basics - a one pager. https://github.com/tazjin/nix-1p#nix---a-one-pager
diff --git a/README.md b/README.md
index 14a970c..ad610cb 100644
--- a/README.md
+++ b/README.md
@@ -403,10 +403,3 @@ Remove leftover EFI entries of removed generations:
 ```shell
 sudo /run/current-system/bin/switch-to-configuration boot
 ```
-
-## References
-
-Some references to websites that helped me create this repository:
-
-* Simple NixOS flake template for NixOS with HomeManager. https://github.com/Misterio77/nix-starter-configs#readme
-* NixOS guide (development enviroment, gaming, etc). https://github.com/mikeroyal/NixOS-Guide#table-of-contents
diff --git a/modules/sendmail-mta.nix b/modules/sendmail-mta.nix
index 0d0fd54..a7c6c06 100644
--- a/modules/sendmail-mta.nix
+++ b/modules/sendmail-mta.nix
@@ -18,7 +18,7 @@
   #   sudo smtpctl show status
   # Send an email.
   # TODO: One has set the sender explicitely to `-f langbein@mail.de`.
-  #   echo "Subject: hello" | sendmail -f langbein@mail.de daniel@systemli.org
+  #   echo -e "Subject: Test\n\nHi there, this is some test." | sendmail -f langbein@mail.de daniel@systemli.org
   # Queue handling
   #   sudo smtpctl show queue
   #   sudo smtpctl schedule all