When you import a certificate into Outlook (or any email client) it allows Outlook to sign your messages digitally, allowing the recipient to verify that the message came from you, and that it has not been tampered with. The message is not encrypted at this stage.
Included with the digital signature is your public key. When someone with their own certificate receives your message they can verify the authenticity of your message and extract your public key. When they reply using their own digital signature you can verify that their message is authentic and extract their public key.
Now, both parties are in possession of the other's public key. The key feature of Public Key Cryptography is that messages encrypted with a public key can only be decrypted by using the private key. When you send a new message to your contact it is encrypted using their public key. They can decrypt it using their private key, which only they know. Similarly, further messages from them to you will be encrypted using your public key and only your private key will decrypt them.