Sorry for people who was reading. I was ultra mad and i deleted my wall of text
here in a smaller text.
It is possible to disable the portal, tips, cute email, email pointing into portal, everything for the end user that would notice the presence of jira it desk.
It would be faster, cheaper and easier to just do all your tracking with email inboxes. If you turn off most of Jira's functions, you don't really have any use for it.
I think you need to step back and look again at the actual requirements you have for request tracking. If it really is "just email", just use email inboxes.
You are not going to make Jira look like OTRS - they are designed very differently to do similar things.
You can run Jira without any UI, over email only, but you are going to struggle a bit, as you won't have access to most of the more useful stuff - the custom fields, reports and so-on.
I would also strongly suggest you look at Jira Service Desk - this is designed to be a helpdesk, and it provides a very simplified "portal" (And is also capable of handling purely email requests). It also has an "agent" licence model - you need to licence agents for it, but the "customers" (including email only ones) are completely free - I know of places with 10,000 customers and 15-20 agents.
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