I just signed up for Sindbad a few weeks ago, no trouble at all putting in my email address. Have you tried a different browser? How about typing the email in a different program, then copy+paste into the field?
Of course, Oman Air's IT system is pretty quirky anyway. The main reason I signed up for Sindbad is that they were never crediting my Oman Air flights to their Etihad partner, even after they said they had. Plus, I am never able to purchase flights online directly from the WY website because they don't take Amex and CapitalOne doesn't use Verified By Visa, so the charge won't go through.
But the airline itself I quite like, and the country I like even better! So it's worth the hassle.