In the case of bounces to a large number of email addresses at the same domain - it may indicate that your emails are being blocked by the ISPs receiving server.
ISPs and other mail systems check incoming mail from each source and can see if you are sending to a lot of addresses that are bouncing.
If they detect that you are then they will start rejecting your campaigns on the assumption your list is made up of automatically generated or purchased emails.
In other words, they may be blocking you on the assumption that you are a spammer.
Our advice would be to contact the ISP that handles this mail to ask them to unblock you as a sender of email.
We would also recommend that you set up SPF (Sender Policy Framework) and DKIM (Domain Keys Identified Mail) records in order to make sure your email is received. This is done via your Domain Name System (DNS) host for example, 1 & 1 or GoDaddy etc.
Please contact our Support Desk so that we can provide you with these details from our email platform so that you can add both SPF and DKIM records to your domain name host.
To read more about setting up SPF and DKIM records please click here.