A hard bounce is an email that could not be delivered because the recipient's email address is invalid or the recipient's email server won't accept emails. There are other reasons why a hard bounce happens, but the above 2 reasons are the most common ones.

A hard bounce indicates a permanent failure. We remove hard bounced email addresses from our platform in order to maintain our mail server reputation and also our user's reputation (you).

A soft bounce is an email that couldn't be delivered because of temporary reasons. When an email address soft bounces, we will continue to attempt to deliver the email for another 2 times. If the email still soft bounces after we've attempted delivery for those period, it will be logged as a soft bounce on the user's report.

If an email address continues to soft bounce across multiple email sends, the address will eventually be considered a hard bounce and cleaned from user list. We'll allow 3 soft bounces for an email address before converting a soft bounce into a hard bounce.

While there are many reasons an email address may soft bounce, below are some common reasons this could happen.

  1. Mailbox is full (over quota).
  2. Recipient email server is down or offline.
  3. Email message is too large. 
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