Friday's crash of Harvard's e-mail system caused the irretrievable loss of mail for approximately 3,000 people, officials announced late last night.

The computer system crashed around 10:30 a.m. Friday when Harvard's main e-mail server, known as HUSC, went down and did not come back up until nearly six hours later.

When the Harvard Arts and Sciences Computer Services (HASCS) revived the machine around 4:30 p.m., the crash had been so severe that many users' inboxes had been deleted.

HASCS makes back-ups of everyone's e-mail in case of crashes, and workers originally thought they could retrieve all lost mail from the backups, Director of Residential Computing Support Rick Osterberg '96 told The Crimson Friday night.

But last night, HASCS officials said they believed 20 percent of its approximately 15,000 users lost their in-boxes. HASCS will be able to retrieve some of these users' mail but is unlikely to recover any mail received between 9 a.m. Thursday and 10:30 a.m. Friday, Director of HASCS Franklin M. Steen said last night.

"We did not realize that e-mail would be lost until Saturday night after our staff had spent all day working on the system," Steen said in the announcement last night.

HASCS ran a back-up of the system on Thursday morning and was running another at the time of the crash, and, as a result, most mail sent between Thursday's backup and Friday's crash was lost.

After the crash, users' incoming mail was queued up on another server and as a result was saved, Osterberg said.

Students with missing inboxes should send a brief e-mail message to HASCS at help@fas. HASCS should be able to retrieve all mail sent before Thursday at 9 a.m., according to Steen's announcement.

After the machine crashed on Friday, HASCS and Digital Equipment Corp., which manufactured the machine, decided to rebuild the system file. HASCS replaced the system file with a more up-to-date version that will help with crash retrieval in the future, Osterberg said.