For the past couple of weeks, all notifications/emails from the community, support and other Marketo sources weren't being delivered into my inbox. This was very odd as I receive many each day. After checking with out deliverability team, they informed my of the following - and therefore, these emails were sent to quarantine. Per our deliverability team:
"Marketo was blocked by the following Transport Rule: Due to recent hacking attempt on Avanade using very sophisticated email technique, we had to block all domains which don’t provide SPF properly and comes up as “Temp Error”. We put rules in place and that blocked these emails because client has SPF record issue. For now, we have whitelisted Marketo but someone should work with Marketo and ask them to maintain their SPF record properly."
So if anyone from Marketo is reading this, please pass this info on to your deliverability team. I will also open a Support case with this information.