No difference. e-mail addresses and domain names can be typed in upper and / or lower.
www . PCaDviSoR . co . UK
www . pcadvisor . co . uk
www . PCADVISOR . CO . UK
are the same
The only thing that matters is the spelling - not the case.
Many people state that their e-mail address is "all lower case" but if makes no difference.
The only time it matters is in a forum when UPPER CASE is considered SHOUTING and RUDE!!!!