Replying to or forwarding a message that was signed (or encrypted) with an unknown key can block the whole application for minutes, as if it's "hung", while the keyserver is queried.

Reproducible: Always

Steps to Reproduce:
1. Receive a message signed or encrypted with a key that is not known locally
2. Select the message in the message list
3. Hit 'f' (F) or r to forward the message or reply to it

Actual Results:  
On OS X, the "beachball of death" appears quickly without any other sign of progress, indicating that the application is no longer handling events. If the configured keyserver is slow to respond for whatever reason, this situation persists until the lookup succeeds, fails on the server, or times out. It is only at that time that the composer window opens.

Expected Results:  
When selecting the message in step 2), its contents are shown in the viewer pane (if it's just signed, of course). The key lookup and ensuing validation take place in the background.
The same thing should happen when replying or forwarding; I see no need to wait for validation. In any case the application shouldn't hang, and provide a way to interrupt the ongoing lookup (probably just by closing the composer window).

I'm marking this as "software hangs" because I have no way to be sure that the software will NOT hang if the keyserver never replies.