Well I hope someone will step in with a better answer than this, but as far as I know if a domain registered with Network Solutions isn't renewed, it is redirected to resalehost.networksolutions.com, and/or underconstruction.networksolutions.com in the hope that someone may renew or buy it.
Perhaps you've got outgoing email to someone at a domain which has just expired, or you've typo'd an email address such that the typo is a recently-expired domain.