Hey Anna,
I've seen some customers report problems when they use Proofpoint, where they blacklist us for short periods of time. This is usually because some people are just flagging WF emails as spam (not sure if on purpose or on accident). It takes a few days of us working with them to get emails working through them again.
If that's not the case for your company (you don't use Proofpoint) then please open a support case so we can check and confirm that the emails are being sent from our end.
Thanks!