Microsoft, which claims more than 110 million active Hotmail accounts, heralded the upgrade as an improvement on the free e-mail site's user interface and e-mail filtering and spam-control tools.
But after launching the upgrade and an accompanying press release late Tuesday night, Microsoft almost immediately withdrew it.
"We started updating last evening and ran into a couple of issues," said Sarah Lefko, product manager for the Microsoft Network, or MSN. "In an hour or so we rolled it back and fixed them and now are waiting for the non-peak times later this evening."
Lefko declined to specify what problems caused MSN to abort the upgrade.
Hotmail originally ran on Solaris servers made by Microsoft archrival Sun Microsystems. Since purchasing the site, Microsoft has switched over to Windows 2000 on a "large portion of the site, with some legacy systems from when we bought the site, including Solaris," Lefko said.
With the future upgrade, Hotmail has simplified the spam-control options available to account holders, who will be able to choose between "high" and "low" levels of anti-spam filtering. An "exclusive" option will only permit e-mail from correspondents listed in the Hotmail address book.
Account holders will be able to view their five most frequent correspondents when composing and addressing mail. Cosmetic changes include the replacement of Hotmail's buttons and icons and the introduction of tabs in the style of the MSN Explorer interface.
The upgrade will add Swedish and Dutch language options, bringing the number of Hotmail languages to 12. Currently, the Web site offers simplified and traditional Chinese, English, French, German, Italian, Japanese, Korean, Portuguese and Spanish.