Will my email ever be sent as plaintext if there's a key for the recipient?

No, not if the GoodCrypto server has the recipient's key.

If GoodCrypto detects any error encrypting a message, then the message is automatically returned to the sender with an explanation about the error.

Additionally, the mail administrator can set a flag so no one can ever send email to specific contacts unless the mail is encrypted. This allows the administrator to add contacts who need privacy ensured before a key is ready.