Updated FAQ

FAQ.md

@@ -676,8 +676,10 @@ it might be necessary to disable battery optimizations for the OpenKeychain app.
 in the additional permissions of the Android app settings of the OpenKeychain app.
 Without this permission the draft will be saved, but the OpenKeychain popup to confirm/select might not appear.
 
-FairEmail will send the [Autocrypt](https://autocrypt.org/) headers for use by other email clients
-and send received Autocrypt headers to the OpenKeychain app for storage.
+FairEmail will send the [Autocrypt](https://autocrypt.org/) header for use by other email clients,
+but only for signed and encrypted messages because too many email servers have problems with the often long Autocrypt header.
+Note that the most secure way to start an encrypted email exchange is by sending signed messages first.
+Received Autocrypt headers will be sent to the OpenKeychain app for storage on verifying a signature or decrypting a message.
 
 All key handling is delegated to the OpenKey chain app for security reasons. This also means that FairEmail does not store PGP keys.