In the first instance, you could use an email client (such as Thunderbird, or Pegasus) and write yourself a "Message Filter" rule to move these unwanted messages, as they arrive in your INBOX, to your Trash/Deleted folder. You could also set the program to delete the contents of the Trash/Deleted folder on exit.
To improve on this, you could enable your email client's Junk/Spam filtering tool, and then change the previous filtering rule, to mark the unwanted messages as SPAM and then move them to your Junk/Spam folder (instead of Trash/Deleted), so that the email client 'learns' what these SPAM messages look like, and eventually filter them into Junk/Spam on it's own.
Assuming your computer isn't hijacked with malware (and isn't actually sending hundreds of messages), these sort of deluge of undelivered message returns, will often just go away after a few days.
Many years ago, I once switched my computer on one morning and had more than 15000 of these being downloaded, and was receiving maybe another 100 every minute. My email client wasn't at all happy trying to clear this backlog on 6Mbps ADSL, but using message filtering rules and built in Bayesian SPAM filtering, my inbox has been 99.9% clear of junk ever since, despite frequently receiving approximately 3500 unsolicited message most days.
