Actually it's not Slackware's bug, but since i used Slackware, it would be great idea to post it here, just in case other people might experienced this too.
I'm using Firefox and FireGPG extension to sign all my mails. The problem was that i always had a wrong signature every time i sign my message. This happened since few months ago when i changed the GnuPG version from gpg 1.x into gpg 2.x. I set the gpg path in FireGPG preference dialog into /usr/bin/gpg2 which is the executable file for GnuPG 2.x. In fact, this is wrong. Today i discovered, that it should be /usr/bin/gpg and we should make a symlink on /usr/bin/gpg to point to /usr/bin/gpg2 and now it is working normally again.
The latest version of FireGPG inspired me to solve this bug because i thought it was FireGPG's bug
Oh yeah, i have changed my digest algorithm to SHA512 instead of staying with SHA1 which is no longer recommended by some people. SHA512 is quite strong for now and for years from now, so i'm using it on my email.
You can get my public key here or here or by looking at public keyserver at PGP Global Directory.