If the return address is invalid, then it's too likely to be spam.
Spammers killed vendor email support and email as a whole so maybe it's time that only email that is properly formed gets through.
I have not received an email since Friday evening. I use a personal email address and emails addressed to my personal email address are forwarded by dotearth.com to my isp provided email account. On Friday secureserver.net (a part of Godaddy.com, provides the email services for my isp) changed their email server to reject emails that do not identify themselves in a way that secureserver.net likes.
Many email servers block emails from known spammers. As spammers have tried to get around the blocks they have changed their tactics. One thing spammers have done is to either not identify themselves or use a false identity. Dotearth.com provides their ip address as there identity. This has been a common practice as long as there has been email. Secureserver.net has decided to arbitrarily apply their standard. As a result, all emails sent to my personal email address, which are forwarded to my isp email account are blocked.
When I talked to people at secureserver.net they say they won't/can't remove the block until dotearth.com changes the way they transmit the messages. This seems arbitrary and capricious.
It seems to me that, in the war against spam, we have lost sight of the purpose of email, mainly to facilitate communication, and that collateral damage has become ok. Secureserver.net, and all email service providers, should keep in mind that delivering messages is their primary task, not stopping spam.