This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
When the email bounces, it might indicate a bad email address or another
problem with either of your email accounts. Before you panic, there a couple
of simple things to try. First, use the Ask Seller a Question link on the
auction page to attempt contact. This message comes from eBay's servers and
is less likely to be blocked by anti-spammed mail systems. If the message
bounced because of a full mailbox, wait a day before attempting to send
If you don't think you'll be able to make email contact, request the user's
contact information using
http://cgi3.ebay.com/aw-cgi/eBayISAPI.dll?MemberSearchShow from eBay and
give them a call. If the email account is not valid or the contact
information is wrong, contact email@example.com.