Lots of requests for the POST /wp-admin/admin-ajax.php?action=send-email

If it’s always the same IP, proxy, or user-agent (and not a common normal browser user-agent), just block it entirely in .htaccess: https://codex.www.ads-software.com/Combating_Comment_Spam/Denying_Access

If not, then I believe https://www.ads-software.com/plugins/block-bad-queries/ will help mitigate that.

This was done by ~50 ips so I’m trying to sort them out so I can block them but did they try to post a comment or just sending dummy traffic to kill my servers?

Based on the query string, it sounds like they are attempting (unsuccessfully due to the 503) to send email from your server by way of WordPress, or to put that another way, hijack your site to send email spam.

This was actually addressed in WordPress 4.7.1 https://www.ads-software.com/news/2017/01/wordpress-4-7-1-security-and-maintenance-release/ but that of course doesn’t stop bots from trying.

Have you tried the plugin mentioned earlier?