Mail Queues
Pressing send is just the tip of the iceberg in relation to the steps involved in getting to the inbox.
The first step is for emails to be organised into mail queues so the mail server can process the workload, make connections to the destination mail server and make a delivery attempt.
Virtual MTAs
A Virtual MTA (Mail Transfer Application) in simple terms is the IP address being used to send email.
Shared IP addresses have no control over which IP address gets used and for dedicated IP addresses the selection can be made within the Email Profile edit screen.
Queued Email
It's not always possible to deliver email on the first attempt because of intermittent issues such as the destination mail server being busy, IP or domain is blacklisted etc.
The majority of email will get delivered on the first attempt and for those messages that don't they fall into an automated retry pattern. Retries are based on the mail server response meaning that 'bad mailbox' would not get retried but 'mail server busy' would.
The default setting for email delivery is to re-attempt delivery for up to 2 days but can be customised where dedicated IP addresses are being used.
Active Connections
Mail queues take a dynamic approach to email delivery in order to maximise throughput and to minimise temporary deferral by destination mail servers.
Connection and retry frequency vary based on mail server response but it is common for mail servers to deny active connections for a period of time. When this happens the queue status gets set to 'Deferred' which essentially means email delivery will be temporarily delayed for a short period.
When a mail queue is in 'Deferred' status email delivery will recommence automatically at the earliest available opportunity.
Destination Mail Servers
Large mail providers such as Google and Office365 provide email services that allows individuals and businesses to use their own domain e.g. Sending an email to someone@abc-company.com may well be using a mail server provided by Google.