Email warmup is the process of gradually increasing the number of emails sent from a new or recently changed email domain or IP address in order to build a positive sending reputation and improve deliverability. This is done to avoid being flagged as a spammer by email service providers (ESPs) or internet service providers (ISPs).
When a new email domain or IP address starts sending large volumes of email, ESPs and ISPs may flag it as suspicious and may block or filter a significant portion of the email being sent. To avoid this, email warmup involves sending a small number of emails at first and gradually increasing the volume over a period of time. This allows the ESPs and ISPs to see that the email is being sent from a legitimate source and to build a positive sending reputation.
The warm-up process usually starts with a low volume of emails (e.g. 50-100 per day) and increases gradually over a period of time until the desired sending volume is reached.