Unfortunately, the Slack invite has to be sent to you by them, there's no automated mechanism yet (they have been saying they will look into one for ages), even I have never received the invite. But you can just keep trying to bug them on Support and on their Social Media, unfortunately the only way these days to get your problem resolved.
What email provider are you using? It's strange that marking as spam blocks it, I don't know any provider that works like this. You've already added to contacts and cleared your spam inbox. Can you try "whitelisting" emails and domains bittrex.com?
OR, use your Bittrex account settings to add a new email? Until problem is resolved?
i'm using yahoo
no idea that happened not only on bittrex mail
but another exchanger mail too
after marked as spam and no more mail from them
its strange
mark as spam should be moved to spam folder
its quite frustrating they no have good solution and really slow
well i will try to send another ticket then
btw, u can't add new email on bittrex
maybe should asking their support for that action