Thanks for the information Till on the capabilities of amavisd-new. In the small amount of reading on the topic, it looks like it would be a simplier solution than my current configuration with dkimproxy.
However, the root issue I have is that some email clients are not able to send to port 587 on the email server. Authentication fails, according to the logs, for some reason.
Does anyone have suggestions how to troubleshoot the cause of this error?