Until the misconfigured filter was corrected Tuesday, customers could not send email to users with .ru domains.
Comcast is one of the nation's largest cable television distributors of porn movies, adult channels and pay-per-view programming. It earned an estimated $50 million from porn last year.
Internet Service Providers use filters to block junk email from coming in and to prevent spammers from sending messages.
Comcast, which last month made a bid to buy The Walt Disney Co., had applied the filter after seeing a large volume of emails sent with return addresses ending with the .ru domain.
Comcast said incoming mail to customers wasn't affected and that spam filters often go awry, but more often incoming messages are the ones mistakenly labeled as spam.