View the original community article here
Last tested: Jan 15, 2021
Solution: As the error says, just wait a little bit. You're allowed to run 5 requests every 5 minutes:
Reason: The SMTP page can be used maliciously as a port scanner, so we decided to limit the frequency with which users can edit that info. Nobody should really need to change it that much. For security reasons, we won't share the frequency limit or lockout length.