You are trying to send an encrypted e-mail, and decide to look up the recipient in the DFN-PKI LDAP address book.

You specify that your search should be inside the ldap-DFN-PKI address book and you want to make an extended search.

Your search criterium is "RA Postfach RWTH" anywhere inside the "Common Name" field of the DFN-PKI user certificates.

You have found the recipient you are looking for, you now need to save them in your own contacts.

Please note the user added "(DFN)" part in the Name. This helps you distinguish your contact as one imported from the DFN-PKI LDAP address book, i.e. with a user certificate (and a public RSA Key associated to it).

You're back to wanting to send an encrypted e-mail, to the contact you previously explicitly found in the DFN-LDAP address book and saved as your "own" contact.

You need to search among your own "Contacts", because you can only send an encrypted e-mail to an "own" contact, choose the one you tagged with "(DFN)".

Choose to "encrypt".