Just the case of the letter (Upper versus Lower) in the domain name (the part after the @ sign and in a web address it would be after the http:// and before the NEXT / ) will not make a difference directly. It might be the difference of the emails that someone is sending you and how the configure it to send you a fraudulent email.
In a website Case is an issue in the PAGE only. For example if you goto:
http://pgardner.net/whois.php?ip=tnt.com
versus
http://pgardner.net/Whois.php?ip=tnt.com (notice the CAPTIAL W in Whois)
the first will work, the second WILL NOT work.
Again, if you goto:
http://pgardner.net/whois.php?ip=tnt.com
versus
http://PGARdneR.Net/whois.php?ip=tnt.com
They will both take you to the same place.
Now there are other ways to exploit the web addresses and I won't go into detail here about it, however, you should NEVER click on a link in an email to get to a website. ALWAYS type in the web address directly in your browsers address bar such as escrow.com or paypal.com. If you click on a link, scammers tend to mask the real location of the link using HTML (That's why I always ask for my emails in PLAIN TEXT. )
If you cannot access a site other than clicking on a link in the email, then perhaps you should consider using a different site.