Emails not sent, but says they are?

In the year or so I have used Qualtrics I have been used to the 'send preview email' feature taking a long time to send (sometimes 2+ hours) or some times not at all.

Today we have sent a survey to 800 recipients including ourselves, and these emails have not been received. It has been over one hour now. This is the first time it has happened with a survey. The Distribution screen says emails all sent, as does the download history csv file. There are no responses either.

Is this something anyone else has encountered? We may be asked to send this again and do not want to chance recipients will reecive it twice. Help!