So we’re talking email sending here, yeah? Not receiving?
In that case the most likely problem is that after enabling Cloudflare proxying the DNS lookup of your domain, example.com now returns the IP addresses of Cloudflare’s servers which are sat in front of your AWS setup. This means if you are using an SPF record which says ‘allows email from this specific host’ then when recipients look up that host to see if its allowed to send email, it isn’t the right IP.
Normally that’s the case if you’ve got SPF records like this:
The Cloudflare proxy only supports HTTP and HTTPs, and those protocols are only supported on specific ports.
If you have a host that needs to use other protocols, such as IMAP, POP, SMTP, SSH, etc, you need to set it to DNS Only or the connections will drop at the Cloudflare proxy since they are not HTTP or HTTPS.