I understand that you have a non-trusted certificate that has been installed that you'd like to be able to use for Mail on your iPhone. I'm happy to provide some help for this issue.
Based on what you've described, it sounds like you meet the requirements for the below article to be of help:
Trust manually installed certificate profiles in iOS - Apple Support
If this is not the case, please let us know exactly what the message reads that you are seeing. In addition, let us know if this is a work Mail account that may be associated with a profile. If it's not, let us know what email provider you are using so we can help look into this further.
Thanks for using the Apple Support Communities. Cheers!