When an SMTP relay server is configured, the web UI will display a "Release" button and allow a message to be manually relayed via the SMTP server to selected addresses.
@see #29
This enables a SMTP server to be configured, and messages to be manually "released" via the relay server. Aditionally, messages can be auto-relayed via the SMTP server do Mailpit acts as a form of caching proxy.
@see #29