Sometimes emails end up in Spam filters or Junk mail boxes for various. You should have your client double check their spam box. If you find that you are having this trouble and you have chosen to change the default "From" address that is attached to the emails ShootQ sends, you may want to make sure you followed all of the instructions for doing that properly. Having this advanced deliverability option set up incorrectly is a common reason for emails failing to deliver.
Another reason clients might not be receiving emails, especially if they used to receive emails, is that they may have opted-out of receiving emails from ShootQ. Emails sent through ShootQ have an option at the bottom to "opt-out" of all email correspondence. This is a legal requirement for applications that send email. When someone chooses to opt-out they are placed into a "blacklist" in ShootQ, and emails cannot be sent to people on this blacklist.
Sometimes, a photographer's client will opt-out from emails from their photographer by clicking on that link and entering their email address. If this happens, then a warning is displayed in the Correspondence tab of that Shoot when emails can't be sent to one or more recipients because they have opted-out of communication. You can view the specific email address affected when you click to view the email.
The only way to remove someone from a blacklist legally is to have them email the organization that controls the blacklist. In this case, that's ShootQ! So, if you have a client who accidentally opted out of correspondence and needs to be removed from the blacklist, simply have them email "firstname.lastname@example.org" and we will remove them.