We often see issues with customers using Apple devices being blocked by our firewall. This is because when Apple Mail is set to automatically detect settings, it periodically examines various ports on the server to see if they are open. The behaviour however is quite aggressive and will often lead to our firewall blocking access from your IP address.
The setting that needs to be disabled is "Automatically detect and maintain account settings". This is normally located in 2 places (Mail -> Preferences -> Select Account -> Advanced) and also under the 'Advanced' tab in the SMTP server settings. If you disable this then it should stop the mail client from probing our servers and then causing you to have your IP address blocked.
This setting needs to be checked on all devices that you may have connecting to your mailbox(es)
This issue has also been observed due to connections from iCloud Notes, so you may also need to check that there is not a profile defined within there connecting to your mail server.
If you cannot find which application on your device is making this connection, you may wish to try using 'Little Snitch' which can identify and also block this for you: https://www.obdev.at/products/littlesnitch/index.html