I am having the same problem. We run MS Exchange 2010, Outlook 2007.
I see the problem from multiple senders attempting to PGP encrypt emails to me. They have all imported my public key into their local keyring. The same key is on their organisation's keyserver - I uploaded it myself, and have checked it several times. When they send me an email marked for PGP encryption, it only occasionally works. Same public key available to all, same sending organisation, same recipient. Several senders have the same problem.
Assume for this example that someone has sent me one that is successfully decrypted in Outlook at my end. They then send me another, sometimes 2-3 minutes later, it can't be decrypted. The error in the log is as reported above - "MAPI Proxy: Decryption aborted". When I try to open the pgp attachment from the email in the PGP Desktop Viewer, it reports that the email was encrypted to two unknown keys and that I have no private keys available to decrypt it.
At other times, I receive an encrypted message, it is successfully decrypted in Outlook (using the MAPI plugins, presumably), I then reply to the sender. It is successfully encrypted and they can read it. But if they then reply to my email again, 9 times out of 10, I can't read it. It can't be decrypted due to exactly the same error as that reported above.
Additional data logged in the verbose PGP log during such decryption failures (the items in square brackets I have replaced for privacy reasons):
17:45:59 Email Verbose Looking for account data for mail server EMAIL; Account ID /o=[my company]/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=[my name]; user [my username]; and address [my email address]
17:45:59 Email Verbose Existing entry is [my email address]
This problem is hugely frustrating as I use the tool all the time to communicate securely with an external organisation.
Any ideas?