Definition
Gradually ramping up the volume sent from a new domain or mailbox so inbox providers build a positive sending reputation before full-scale outreach begins.
Inbox providers treat a new domain with no sending history as a risk. Sending hundreds of cold emails from it on day one reads as spam behaviour and can get the domain blacklisted before a single real prospect sees a message, which is why agencies stagger volume up over several weeks instead.
Where this comes from
Read: Cold Email Deliverability: Why Your Emails Land in Spam (And How to Fix It)