If your using POP3 or IMAP, make sure the ports are set correctly. This can be verified in cPanel, goto "Mail Accounts" , on your email account, select "More" and then "Configure Email Client"... Near the bottom of the page are all of settings needed to configure on your device.
Also, check for any errors with SSL when logging into webmail (or cPanel) via HTTPS... I've seen issues with providers using a self-signed certificate or an expired certificate that fails quietly on a mobile device, but gives some warning or error in webmail (or cPanel)... since the same SSL certificate is used for email, cpanel and webmail...