Outlook+PGP5.5.5 cannot read Evolution+GPG emails.
Outlook+PGP5.5.5 can read kmail+GPG emails.

The default options given to GPG for signed or encrypted mail are not readable by recipients using Outlook.  They appear as unknown MIME ATT000???.dat attachments, instead of as inline text encoding, even if a ~/.gnupg/options file includes the "textmode" option to force PGP compatibility.

Step by Step:
 * ensure gpg is installed with keyring
 * compose an email to any recipient with a key
 * mark Security/GPG Sign or GPG Encrypt options
 * send the mail
 * use Outlook with PGP to receive the mail
 * note ATT000???.dat attachments
 * Content-Type: multipart/encrypted;
   protocol="application/pgp-encrypted"

This MIME type is not supported by Outlook.  The contents of the attachments are the textmode encrypted data or signature, so a cumbersome workaround for encryption is to save the attachment, rename it to .asc, and then ask PGP to decrypt it.  This does not work for verifying signatures, as I was unable to save the message itself with the proper checksum to be verified separately.

Conversely, other emailers such as kmail and Outlook will replace the original message body with a textmode encrypted body.  Mails formatted in this way are read and handled properly by Evolution.