I received an email regarding Dell Laptop Warranty Extension from this domain @dellteam.com. The details show that it is mailed and signed by dellteam.com. The email asks to reply with the Service tag of system and Contact Number. I don't know if it is genuine email from Dell or it is a fake/spam email?
You're assuming the email was legitimate, and not forged/spoofed.
ANY request for a service tag OTHER than from a call or email YOU initiated, is suspect. There are way too many phishing scams these days to trust any unsolicited request for information - whatever the whois lookup tells you.
Most people can't or won't parse headers on email to locate forgeries -- so the best strategy: delete all unsolicited requests for information.
It's almost certainly fake -- if it were from Dell, they'd have the service tag.
If you want an extended warranty, call Dell for a quote on one. However, note that buying an extended warranty on a system you already have is rarely a good value for the dollar - what costs $100 or so at purchase will cost you 3-4 times that as an extension. You're essentially paying in advance for a major repair - you're better off banking the funds if a repair is needed.
DellTeam.com is CERTAINLY a Dell e-mail domain.
Anytime you call Dell support, they identify your computer by asking for your Service Tag (serial number) or Express Service Code. So, that's normal.
If you every want to find out who owns a domain, it's really quite easy --
If you do a WHOIS lookup for dellteam.com you will see: