After Microsoft mandated compliance with its Sender ID email verification program this week, some emails - mostly from small-to-midsize firms - sent to MSN Hotmail are being sent to junk email boxes, reports DM News. Specifically, emails from companies that have not published their Sender Policy Framework (SPF) are being flagged with a "Sender Unknown" question mark while a "safety bar" within the email reads, "The sender of this message could not be verified by Sender ID."

More than 1 million domains have published their SPF records, according to Microsoft. According to one deliverability expert, MSN is providing the sender-unknown warnings when "there is a hard fail, or in situations where the spammer is making up a domain name."

In its offensive against spam, Microsoft has released other tools for Hotmail, VNUnet writes. The software giant released a preview version of Smart Network Data Services, which offers statistics about emails being sent to Hotmail accounts - it shows the volume of email coming from single IP addresses and whether Hotmail's filters have tagged it as spam. ISPs can identify accounts used to distribute spam emails, either deliberately or because they are infected by a virus and have been turned into "zombies."

Microsoft has also launched MSN Postmaster, a website where ISPs, email service providers and legitimate senders of bulk emails can learn about spam issues.The program is intended to educate senders on how to prevent legitimate emails from getting caught in spam filters.