domain validation for e-mail addresses

I've just started playing with gethostbyname() in hopes of validating domain
names on e-mail addresses we've collected (with people's permission <g>).
These are mostly keyed in either by the customer on our web site or by
customer service people.  Strangely enough, the addresses are not always
valid.  We were hoping to correct the obvious typos and misspellings. 

I've been running a sweep over our master file checking the domains.  I've
noticed that some domains like us.ibm.com show up as invalid. Ping and
traceroute also give the same result.  Since us.ibm.com is something I've
seen a lot on this list, I'm curious how this gets resolved into the correct
domain name.  I notice on the internet headers from messages from people
whose address is @us.ibm.com that the domain part is a bit more complex than
it is in the from line.

TIA
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