Weird Exchange issue with Mail

[ashman70]

I have a user who is still on El Capitan using mail for several email accounts, one happens to be the corporate Exchange email. They travel between three locations, all locations are connected via an internal network. Only at one location do they have issues where mail shows offline for the Exchange account, when I click online, it says downloading messages but they never download. At the other two offices they don't have this problem. I've checked the firewall and mail is allowed out on port 443 so it's not that. It's just so weird. Any thoughts? The user wants to stick with mail where they have all their accounts so switching to outlook isn't really an option. Also the user has like less then 8GB of free disk space so upgrading to High Sierra isn't in the cards at the moment.