That's not typically how DDoS attacks work. The entire domain under attack is brought to its knees when it happens, and that would certainly include any messaging on any apps utilizing said domain's network traffic, and that includes web sites hosted remotely that might pull data directly from any internal resources within the organization. One thing that *could* be useful to combat DDoS in many cases would be to have multiple mirror sites for failover protection, but I fear in this case it is untenable in even the slightest...