From time to time I send letters to my colleagues, using a specialized service for mass mailing of e-mail.
On average, I send letters to 250-300 recipients. After about two-hundredth user, some emails are not sent with an error:
MX: no MX RESP: 5.1.3 MX or domain found
When you resend – emails are not sent for some time. Apparently blocked. I use DNS cloudflare.com. What could be the problem? cloudflare.com has a limit on the count of requests per time unit? How to avoid?