Bounces, Deferrals, and SMTP Errors
Separate a dead address from a temporary delay, and read the SMTP code before you suppress customers.
Delivery3 min
Bounce Rate vs Complaint Rate: Which Hurts More
Bounces mean the server did not accept the mail. Complaints mean a person marked accepted mail as spam. Which one is more dangerous depends on the provider measuring it.
Delivery3 min
Why Is My Email Bounce Rate Suddenly Increasing?
A sudden bounce spike is usually a new list, a provider block, or a deferral wave. Here is how to tell those apart before you suppress customers or raise volume.
Delivery3 min
Email Deferrals: Why Mailbox Providers Delay Mail
A deferral is a 4xx response: the mailbox provider is telling your ESP to try again later. Here is how that differs from a bounce, a block, and the spam folder.
Delivery3 min
Hard Bounce vs Soft Bounce for Deliverability
A hard bounce is a permanent failure. A soft bounce is temporary. Here is how eCommerce senders tell them apart, what Klaviyo suppresses, and which bounce should stop the next send.
Delivery3 min
Why Purchased Email Lists Damage Deliverability
Bought addresses did not ask for your mail. They bounce, they complain, and they are how pristine spam traps get onto a domain that used to be fine.
Delivery4 min
SMTP 4xx vs 5xx: How to Read a Delivery Error
A 4xx code means try again. A 5xx code means the receiver rejected the message. Here is how to read SMTP errors when authentication already passes.