A communications system may include a client device for generating email processing jobs and a proxy server communicating asynchronously over the Internet with the client device for processing the email processing jobs. The client device may post a given email processing job to the proxy server with a unique job identifier (ID), and the proxy server thereafter may post job results for the given email processing job to the client device also with the unique job ID. The client device may also advantageously post at least one other email processing job to the proxy server over the Internet at a time between posting the given email processing job and receiving the job results therefor.