Though marketers are applauding Microsoft for adding an unsubscribe link to its free e-mail service, some are urging the company to rethink a feature that could result in some wanted e-mail getting blocked.

Some are also urging the company to make it easier to get the unsubscribe link to appear.

Microsoft in July began putting an unsubscribe link in the user interface of some Windows Live e-mails—the new free service replacing Hotmail—so subscribers will quit reporting permission-based commercial e-mail as spam when they simply want to unsubscribe.

Marketers for years have been urging e-mailbox providers to add unsubscribe buttons to their user interfaces because consumers often use “junk” buttons to prevent mailers’ messages from arriving even though they signed up for the e-mail.

Under Microsoft’s plan, e-mail arriving with a valid list-unsubscribe function—a line of code that allows ISPs to automatically forward unsubscribe requests back to the sender—is eligible for an unsubscribe link.

A troubling aspect of the unsubscribe feature is that when a user clicks on the unsubscribe link, the sender—though not reported as a spammer—will automatically be added to the mailbox holder’s “block” list.

And in the fickle world of business-to-consumer marketing, this feature could make it difficult to communicate with former customers who decide to come back.

Once a mailer is on a Windows Live subscriber’s block list, the user must actively unblock that sender if he or she wants to re-subscribe. It is highly unlikely most consumers will know to do that.

“We have people that opt in and out all the time, especially with retailers,” said Tricia Robinson, vice president of marketing and strategy at Premiere Global Services’ marketing automation division in Atlanta. “People will opt in and out of catalogers and retailers based upon their mood and fads, and whether or not they liked the last couple of campaigns.”

A Microsoft representative said the company is considering changes to various aspects of the unsubscribe link before it comes out of beta, and the automatic-blocking feature is one of them.

Also troubling is the narrow criteria Microsoft currently uses to determine which incoming e-mail gets the unsubscribe button.

In order to get the link, the sender must be on the recipient’s “allow” list—addresses the mailbox holder has deemed as safe.

However, mailers certified by reputation firms, such as Return Path or Habeas, should also be able to get the link, contends Joshua Baer, chief technology officer of pay-for-performance marketing services firm Datran Media.

“This [the unsubscribe link] is a great first step, but I hope as they get comfortable with it, they’ll be able to expand it to include more legitimate mailers,” he said. “It is extremely unlikely that even the best brands could get 50% of their list to add them to their address books.”

Baer also said he’d like to see more standardization from ISPs on how to get added to users’ address books. “Right now, the lack of standardization makes it extremely challenging for marketers to get a large number of their subscribers to add them to their address book,” he said.

Microsoft is in the midst of moving its Hotmail address holders to Windows Live. The change reportedly should be completed in the next four to six months.

The unsubscribe function will come out of beta when Windows Live is completed, according to a Microsoft representative.