It's hard to understand the impact of granting exceptions and cutting people slack until you've been on the organizers' side of this situation.
It sounds harsh, but drawing a strict line in the sand results in a better situation for everyone. Once you go down the path of catering to everyone's exceptions or oversights, you start allocating disproportionately large amounts of time to the people who can't follow schedules or manage their time. A 1:1 catchup meeting is far less efficient than a 1:20 orientation meeting. Do just one of these 1:1 meetings and you've doubled your time commitment.
Ironically, the person who didn't read their e-mail gets individual 1:1 attention and has more flexibility on the scheduling. It's almost like rewarding those who fail to follow the rules.
When I handled onboarding/orientations, I thought cutting people slack for missed meetings and deadlines would build rapport and encourage them to be more considerate next time around. Instead, it has the opposite effect. Once you start signaling that meeting obligations are semi-optional and that deadlines are more suggestions than cutoffs, a lot of people start taking advantage of the situation. Sort of a broken windows theory for time management.
On the other hand, if the rules are strict and the consequences are clear, people start paying attention, prioritizing organizational order, and reading their e-mails. You do a lot less hand-holding, and you can spend your time focusing on catering to those who follow the rules instead of doing 1:1 meetings with the people who can't or won't read their e-mail.
Don't let the tail wag the dog. Set clear expectations and stick to them. Grant exceptions when people communicate in advance (e.g. can't get a flight, flight delayed) or have truly unpredictable exceptions (car accident), but don't set the precedent that reading e-mail is optional because you can count on someone else to fix it for you later.