It's all snobbery really, but ones that spring to mind:
Couples:
daveandjane@domain.com
Even worse if it's daveandjane@hotmail.com
Families:
People think they're being so clever with: dave@thesurnames.com
I have an irrational hatred of that one!
But snobbery is such good fun.
I like daveandjnae@hotmail.com.
You're not sure if
(a) they made a typo and don't know how to correct it
(b) they made a typo and just can't be bothered to correct it
(b) they found daveandjane was already taken so they cleverly tweaked themselves an alternative
(c) Jane's parents were really awful at spelling and we should be happy they didn't try for a Gaelic name
(d) the registrar was on her second bottle of Smirnoff by the time she came to enter Jnae's name
(e) Daveandjnae is in fact the second most common female name in Kyrgyzstan
Personally, if I have receive emails of the @gmail, @yahoo, @hotmail, or @lavalovelube.com variety, I bin them and have the originators hunted down by robots.
I've cleverly picked a domain name that is mildly difficult to spell. This is a primary defence against idiots. It does mean I have to india-romeo-india-delta-echo-sierra-charlie-echo-november-charlie-echo every time I'm asked for it. Which earns me a second 'huh' and I have to go "I for igloo, R for..." Usually, I give up and have them killed by robots too.
I have a second main domain which works on the 'yes, cow and all one word' principle.