How to communicate with other mail clients? (evolution, kmail,kontact)

FireGPG is a really wonderful and essential tool, but I cannot make it work to send/receive signed or encrypted mails to/from desktop mail clients like evolution or kmail (kontact).

This topic is for request advices to achieve PGP communication between other mail clients. Is it possible?

The only feature that works for me was receive a encrypted mail in gmail (and did not work fully).
I use kmail 1.9.10 and gmail, but tested with evolution too.

The problems explained:

Receiving mails in gmail
Sign
The kmail client sends the message with the "signature.asc" file attached. FireGPG do not find any signature. I suppose that FireGPG do not support attached signatures.
Encrypt
FireGPG identify an encrypted message. You can decrypt it, but the result is in base64 format, so you can not directly read it, which is very annoying.

Sending mails from gmail
Sign
FireGPG uses inline signs, so the mail contains the original message and a sign at the end. Kmail shows the contents, but do not try to verify the sign, it do not identify any sign in the message.
When kmail signs a message, attach a signature.asc file. Perhaps it require the same on incoming messages.
Encrypt
The same, kmail shows the encrypted text, but do not identify it as an encrypted message.
Maybe any base64 issue like receiving?

Are there any tricks to solve the incompatibilities?
If not, the usefulness of FireGPG is very limited.

Regards.