Time To Inbox is the duration from when an email sent from a sender reaches the receiver’s inbox and is available to read. Once the sender hits ‘send’, the email goes through multiple channels before it lands in the recipient’s inbox.
In order to send email, the email client needs to chat with another program, called as ‘MTA – Mail Transfer Agent’ on the sending server. MTA is the program that’s responsible for transferring emails from one computer to another. Both sender and receiver require an MTA to be running on their email service provider. Once the Receiver MTA verifies that email is not spam, it then delivers the email to the one of the mailbox dedicated for that receiver. The receiver’s mailbox is the place where all mails for a particular user are stored.
In over 95% of cases the above process takes less than a minute i.e. Time to Inbox is less than a minute. In a few cases the message could take as long as 5 days to complete its trip from sender to recipient. It rarely takes more than 5 days as one of the SMTP servers will send the message back as undeliverable.