If a persons emails on hosting please check the following:
- Clear the brute force force firewall and take their IP address and check the config firewall in WHM.
- Check the hosting package is not over quota and also their individual mailbox.
- Is it happening on one device or multiple, mobile phone?
- Has their password changed and needs updating?
- If they're unable to send emails is this to everyone or certain users. Are they getting a bounce back? Can you get a copy of the bounce back?
- Login to webmail and check they can send and receive emails from there. If they can, it's an issue with the setup of the emails on their device/ they're blocked out.