diff --git a/docs/known-issues.md b/docs/known-issues.md
index 0304ed2..282bfd5 100644
--- a/docs/known-issues.md
+++ b/docs/known-issues.md
@@ -30,3 +30,11 @@ echo $fiber->start();
 echo $fiber->resume();
 $fiber->resume();
 ```
+
+## Unsupported PHP Extensions
+
+The following extensions are known not to be compatible with FrankenPHP:
+
+| Name                                                        | Reason          | Alternatives                                                                                                         |
+| ----------------------------------------------------------- | --------------- | -------------------------------------------------------------------------------------------------------------------- |
+| [imap](https://www.php.net/manual/en/imap.installation.php) | Not thread-safe | [javanile/php-imap2](https://github.com/javanile/php-imap2), [webklex/php-imap](https://github.com/Webklex/php-imap) |