I've posted a message via the support widget in the control panel, but just in case it doesn't get through (as it says it sends email) I'm posting here, too. Anyone else see this?!
At this time the US IMAP service is running very slowly: https://status.opalstack.com/incidents/56
When this happens it can cause the webmail server to run out of resources due to a high number of slow requests.
The webmail server handles mail for both US and DE so if you're on DE mail you might be affected by this as well.
Should be working normally now.