It's happening again today. Are there any Marketo issues today with the system?
1. Whitelist: Configure Protocols for Marketo - Marketo Docs - Product Docs
2. Single flow action send (if there is an error, you can catch it in the activity log and troubleshoot further)
I consider IP whitelisting a worst practice and tell all my clients to turn it off. Rest assured that there are many leads in your database who are using the sane anti-spam software you are! This isn't the relatively wild times of the 90s when we would build bespoke anti-spam rules. Not whitelisting is the best way to catch deliverability problems early.
Also be sure that your subject lines are sans punctuation and special characters (aside from the new emojis and such)!
I've heard of several reports of characters such as ( ) { } [ ] \ | / ? > < and more creating blocks for people.
If you mean that you cannot login, yes, it's affecting at least one Pod. Please contact support so they can help investigate.