You must be aware of that what those bouncebacks means , it may be due to your ip got blacklisted or invalid recipient address and so more reasons.
Here , I’m adding some common bounce back error messsages that you may or might see .
1. 421 Server temporarily unavailable. Try again later. IB101
This is because of email queues generally , you need to try after some time or clear out the existing high volume of queue.
2. 550 Recipient not found.
This may be you familiar with . The reason for this error message is that you are sending that email to a non existing email address(recipient email is not valid)
3. 451 Sorry, I wasn’t able to establish an SMTP connection. I’m not going to try again; this message has been in the queue too long.
The above error gets because either you are blocked on recipient side or you are misspelled the recipient email address.
4. 550 domain.com From: Domain is invalid. Please provide a valid From: IB506
Yes . this is an interesting bounceback error , truly this is because the domain hasn’t any valid mx records. you need to verify if mx records are exists.
5. 550 domain.com Invalid SPF record. Please inspect your SPF settings, and try again. IB508
As you see clearly in the error message , The sending email address’s domain has an SPF record that does not authorize the sending email server to send email from the domain.
6. 421 Temporarily rejected. Reverse DNS for this IP failed. IB108
yes. Most of the error messages clearly specify the reason behind of the bounceback as this one.
The IP address attempting to send mail does not have reverse DNS setup, or the DNS lookup failed.
7. 452 This message contains too many recipients Please reduce the number of recipients and retry. IB605
Yes . Obviously you need to reduce the number of recipient addresses to be send.
8. 554 RBL Reject.
9. 554 Connection refused. This IP has a poor reputation on Cloudmark Sender Intelligence (CSI). IB103
10. 554 Connection refused. This IP is listed on the Spamhaus Block List (SBL). IB104
11. 554 Connection refused. This IP is listed on the Exploits Block List (XBL). IB105
12. 554 Connection refused. This IP is listed on the Policy Block List (PBL). IB106
The reason behind this error message is that your IP address is got blacklisted on appropriate spam databse and you need to remove the blacklist by sending delist request.