A soft bounce means that there’s a temporary issue that’s causing an email to bounce. A hard bounce means that there is a permanent reason why an email cannot be delivered. Typically, ESPs will consider a failed delivery attempt as a hard bounce after three soft bounces.
Some common reasons for a soft bounce include:
The email is “undeliverable”— An undeliverable email can be the result of a temporary server outage or the server undergoing maintenance. If this issue persists, the server could be permanently gone.The recipient’s inbox is full— A soft bounce can also be a symptom that your recipient’s mailbox is too cluttered and can’t accept any more messages. This can also mean that your contact has stopped using this email address, so you may want to follow up with them via another channel to update their information in your system.The email fails to meet server requirements— Your message doesn’t meet the recipient server’s anti-spam, anti-virus, or DMARC authentication requirements.There's an issue with message size—Your email message is too big.You receive an autoreply — If your contact is gone on vacation or is otherwise out of the office, your emails to them will bounce.
Some common reasons for a hard bounce include:
The email address doesn’t exist — This can happen due to a recipient misspelling their email, or leaving the affiliated organization.The email address is fake — this may happen if you offer something in exchange for a contact email.The email is blocked — This occurs when the receiving email server has rejected your email. Either your recipient intentionally blocked you, or their domain has stringent spam filter settings.