Today smartphones are computer being able to perform any kind of function.
I don't see much of difference from real computers.
So to answer your question we need to examine what those devices are used for.
If smartphones are used to perform the same kind of functions performed by normal computers, there is no difference between mobile and not mobile security.
The only difference is that we pay more attention in the security of our computers and zero attention in the security of our mobile devices.
I would therefore conclude by saying that the biggest challenge in mobile security is owner's awareness of the extreme need of a mobile security.
The phone is your castle.
Once this is clear then we can discuss about network security, aimed for example to protect the privacy of mobile users from network operators inference in their life.
Everything else that happens inside the mobile device has the same issues of a noun mobile device when it comes to secure its functions.

Are there any email that does not read your content?

There are no free meals.

At the same time though the way internet email protocol is structured does not give you any certainty of privacy.

Meaning, you may pay for it and still have your mails read from someone other than its recipient.

So, unless you send an email which is already encrypted before it leaves your computer, you should not have any expectancy of privacy.

There are, of course, private email services fully decentralized (meaning no servers, no employees that access it nor government that can back door it), fully encrypted ( before data leaves your device and until reaches the recipient device) that fully obfuscate the ip traffic to protect your identity and your location (yes, it is not just about the emails, it is also about your Metadata) and that leave no digital footprints, to avoid any trace back. But those email services work for closed groups and do not allow the exchange of data with standard internet mail services.