E-mail deliverability folk are all abuzz this week about the roll-out of major changes to Hotmail that began Tuesday. It’s hard to accurately predict what impact they’ll have on senders, but we can make some early, educated guesses.

The feature that’s receiving the lion’s share of attention is the one Hotmail has dubbed “Sweep”. Sweep helps recipients to move what Hotmail calls “gray mail” – mail that is not spam, but that may no longer be relevant to the recipient – out of the inbox. Like an inbox janitor, sweep declutters the inbox by automatically moving gray mail to the trash or to another folder for later action.

What makes Sweep different from existing user level filtering tools is ease of use. Recipients won’t need to build cumbersome filtering rules to manage gray mail; Hotmail presents a simplified button interface to set and apply sweep preferences.

That’s good news and bad news for senders. The good news is that recipients will have an alternative to reporting permissioned mail as spam. Senders have long bemoaned that lazy recipients  all too often use the “This is spam” button as a sort of malformed unsubscribe request from permission-based mail. Sweep will move gray mail out of users’ inboxes without a hit to sender reputation. The bad news is that Sweep remembers user preferences, so if a user sweeps a sender’s mail once, Hotmail is likely to continue to sweep that sender’s mail until the user intervenes.

If the recipient deletes a particular sender’s mail unopened several times, Hotmail will eventually prompt them to unsubscribe with a new feature called (appropriately enough) Prompted Unsubscribe. Again, a mixed bag for senders: it will undoubtedly reduce list size by some amount; however, it removes recipients who are no longer engaged anyhow, which should actually improve open and conversion rates.

One final feature of note from a deliverability standpoint is Time Traveling Filters. Hotmail will retroactively filter unopened mail that had already made it to the inbox if the sender’s reputation later tanks. That means there’s no longer a guarantee that a message delivered to the inbox will actually stay there until the recipient can act on it.

Hotmail’s changes emphasize the growing importance that reputation and engagement will play in the e-mail universe. The take-away for senders: don’t send gray mail. Keep your recipients engaged with relevant, compelling content, so that what gets to the inbox stays in the inbox.