Is there an E-Mail client for linux which allows to search old gpg encrypted mails (i.e. find all mails which contain a particular keyword in the body, including the encrypted ones).
Mutt has pretty good PGP integration. The wiki shows what settings you need to add to your
.muttrc; these settings may already be present in the system-wide configuration file (for example, on Debian, PGP/gpg works out of the box).
Mutt supports mbox, mh and maildir mailboxes. If you search in a mailbox that happens to contain encrypted mail, you'll be prompted for your gpg passphrase (if you haven't already entered it in this session either in mutt or in an external keyring program), and mutt will find occurrences in encrypted mails.
Mutt doesn't have a command to search multiple mailboxes. If your mails are stored in single files, you can make symbolic links inside a single directory to make a huge mh-format mailbox; it may be a little slow.
Also, I haven't used it, but Notmuch is a recent tool to manage email that supports gpg and is good at indexing, so it should support searches of encrypted mails.
It has been a long time that I used this, but
pine could do almost anything to mails - there might even have been an option to index decrypted mails (without storing them decrypted).
PGP-pine-integration describes the integration. This is done through a display filter - so, sorry - no luck with pine here, too.