What is the difference between MX sending and Relay sending?
Email Servers can send emails via two methods: 1) Relay Sending – this approach is where your Email Server passes on the emails to another Email Server, who is then responsible for delivering the emails. This approach is like one post office passing on all their letters for delivery to another post office. This approach is normally used for dial-up connections as it’s a bit faster, but provides you with less control and slower (or no) feedback of failed messages. 2) This approach uses the recipient’s domain MX entries to deliver the email directly to the recipients email server. (See FAQ B13 – What is an MX Entry). For example if you were to send an email to anybody@google.com this approach would result in the email being delivered directly to one of the Google email servers. This approach is the preferred choice for permanent connections, as it provides you with more control and better feedback in terms of failed messages. See also FAQ Article B9 and FAQ Article B17 on using MX Sendin
