Oxmail aims to provide a reliable mail relay service. No messages are discarded or filtered.
There are three possible outcomes for each message presented.
As much checking as is practical is performed prior to acceptance of a message to maximise the chance of delivery. For mail from outside Oxford, the recipient address is checked with the downstream MTA. The sender address is verified (for external senders just the domain) in order to help ensure that in the event of non-delivery after acceptance that an error message can be sent to the sender.
Pre-acceptance checking tries to ensure that all accepted messages are deliverable. Any message that cannot be immediately delivered is queued and further delivery attempts are made from time to time. The retry interval starts small and increases with time. A message will be queued for 7 days before being returned to the sender.
Rejection occurs at SMTP time. The sending MTA is responsible for informing the sender of this.