Significant delay/inconsistent email invite delivery

We are experiencing a significant delay and inconsistent delivery for some of our events. Our event was approved this morning ( before 9am PT on July 12, 2023) and it's been a full day (now 4:30 pm PT) and the event invitation has not been delivered to the invitees. Additionally, other ERGs have reported that emails never came through at all.

We don't mind a delay (for batch sends, for example), but the delay needs to be consistent/expected if it happens at all.