Is this a groupcall.com email for Groupcall Messenger? I can see that one of the mail servers for the groupcall.com domain is sendgrid.net and that mail service does operate a suppression list.
What happens is that if there is a difficulty in delivering mail to your mailbox even after re-trys then the sendgrid servers will add your email address to their suppression list and send no more mail. The school may not be aware that their messages aren't being delivered by sendgrid. The school will need to make their customer contact point with Groupcall Messenger aware and ask Groupcall Messenger to whitelist your email address with Sendgrid.net.
The workaround with the school is to give them a new email address but as soon as sendgrid remove your email address from their suppression list then school emails will flow in once more.
