An SMTP relay is a server that accepts email from you and passes it on to the recipients' mail servers. Services such as SendGrid, Mailgun and Amazon SES are relays built for sending large volumes from applications.
Relays and cold email
Relays are made for email people asked for: receipts, password resets, newsletters. Most of them forbid cold email in their terms and close accounts that send it, because complaints from one sender hurt their shared IPs.
Cold email works better from real mailboxes, such as Google Workspace, sending a modest number of emails each, because that's what one person emailing another looks like.
Relays inside Google Workspace
Google Workspace also has an SMTP relay service for sending from devices and apps on a company's own domain. It shares the same reputation as your mailboxes, so it isn't a way around cold email limits.