By default, when using a Telenet address on your smartphone, the Telenet mailserver (uit.telenet.be will not accept messages if they're not sent from within the Telenet network (e.g. when using Proximus, Mobistar or some other ISP or cellular operator).

You can resolve the issue by adding SMTP authentication to your outgoing mailserver settings. Simply add your username and password, and you will be able to send mail using Telenet SMTP.