Create plugin to send messages via XCLIENT

[alexkiro]

We should create a plugin that gets a message and sends it via to a third-party SMTP via XCLIENT

Acceptance criteria

• The plugin must expose an eval rule for this
• The eval rule must accept a recipient as an argument
• The eval rule must accept a server as an argument
• The plugin must accept options that specify TLS certificate

Workflow:

• Resolve the MX record of the server
• Resolve the A/AAAA records of the MX servers
• Attempt to connect to the A/AAAA records in turn, until one is successful
• Send the EHLO command
• Send the STARTTLS command
• Send the EHLO command again
• Send the XCLIENT command with the arguments extracted from the message (NAME | ADDR | HELO)
• Send the EHLO command again
• Send the sender (extracted from the message)
• Send the recipient (set in the eval rule argument)
• Send the message (set in the eval rule argument)
• If the results (after DATA) is 5XX return True, otherwise return False