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.
A good explanation about setting up SPF-records for domains can be found here:
Like any other MTA, Postfix has a maximum message size that it allows to pass through. By default, when not defined, it is set to 10 MB.
To change it, add/change the following in main.cf:
message_size_limit = 15728640
This sets the limit to 15 MB. To make it unlimited, set it to 0.
Got this link from a colleague:
It describes the inner workings of Postfix, the well-known mailserver for Linux.