.mobi is Mobipocket but that's the old Kindle format. It's been superceded by AZW3 and KFX neither of which can be emailed. Emailing .mobi works though.
You can see what formats send to kindle by email supports here.
https://www.amazon.com/gp/sendtokindle/email
Word documents and RTF should support page breaks. That might be easier than trying to create a .mobi file on a phone.