unable to encrypt to one specific user

[MindFuq]

* savanna <savanna@cat.org.au> [2002-11-03 18:44]:
>
> The difference is in the second field - a 'q' for keys that work,
> nothing or 'i' for the key that doesn't.
> 
> Any ideas? I've searched the doco for info on the --list-keys option -
> found nothing.

According to ./doc/DETAILS, "i" means invalid.  

Here's the excerpt:

2. Field:  A letter describing the calculated trust. This is a single
            letter, but be prepared that additional information may follow
            in some future versions. (not used for secret keys)
                o = Unknown (this key is new to the system)
                i = The key is invalid (e.g. due to a missing self-signature)
                d = The key has been disabled
                r = The key has been revoked
                e = The key has expired
                - = Unknown trust (i.e. no value assigned)
                q = Undefined trust
                    '-' and 'q' may safely be treated as the same
                    value for most purposes
                n = Don't trust this key at all
                m = There is marginal trust in this key
                f = The key is full trusted.
                u = The key is ultimately trusted; this is only used for
                    keys for which the secret key is also available.