One of the keys to effective deliverability is to ensure you are respecting the mailbox providers' resources. Spammers often want to send as much email as possible before a mailbox provider catches them, so they open as many connections to the mailbox provider as possible and send a large volume of email all at once. By taking up so many resources, it makes fewer resources available to legitimate senders.
Although setting your Mail Transfer Agent (MTA) to deploy email using these guidelines does not guarantee delivery or inbox placement, it (along with other sending best practices) helps a mailbox provider gain trust that you are a legitimate sender and not a spammer.
Many Email Service Providers already configure their MTAs with appropriate connection and throughput settings. Contact them if you are seeing Simple Mail Transfer Protocol (SMTP) bounce messages indicating throttling so they can work with you to make adjustments.
