Are endorsed undergraduates eligible to submit papers (and eventually get it accepted) to arXiv? Is it possible for an undergraduate to submit his first preprint to arXiv and get it accepted ? Assume that he/she has been endorsed.


I am asking this because so far the papers that I have observed the email id seemed to be email id of Professors of some university or colleges but of course it may not be the case in general.
Yes, of course.  The arXiv staff doesn't sit around evaluating the credentials of everyone who submits.  They reserve the right to reject even endorsed submissions, but this is unusual.

The main obstacle to publishing on the arXiv as an undergraduate is writing a suitable paper and getting it endorsed.