If your IT service provider has set up your firewall in a way that does not allow IMAP or POP3 traffic by default, you will not be able to receive notifications from your YouTrack InCloud instance.The other use case may be an application integrated with YouTrack installed in your local environment and not accessible from the outside.
To solve the problem, the IT provider should know a range of IP addresses from which connections will be coming and create firewall rules that allow traffic from these sources. Amazon Web Services (AWS) publishes a list of IP address ranges in JSON format. To learn how to use this information and subscribe to updates, refer to the AWS documentation.
It's not possible for us to provide you with a fixed IP address for your InCloud YouTrack due to deployment architecture limitation. This is something we're going to implement in future releases of YouTrack.