Soft bounce

A soft bounce comes back with an SMTP error in the 4xx range, or with a temporary failure inside a bounce message. Typical causes are a full mailbox, a mail server that is down for a short time, a message that is too large, or greylisting, where a server asks unknown senders to try again later.

Sending servers usually retry a soft bounce for some hours or days. If the problem clears, the mail goes through. If it does not clear before the retry time ends, the message is given up and counts as failed.

Pay attention to patterns. A few soft bounces are normal. Many 4xx replies from one provider, such as '421 try again later' or a rate-limit message, mean that you are sending too fast or that your reputation is low. Slow down instead of retrying harder. An address that soft bounces again and again over many sends should be treated like a hard bounce and removed from your list.