Is it technically possible to recover "deleted" emails from computers?

...
Administration officials said they could offer no estimate of how many e-mails were lost but indicated that some may involve messages from White House senior adviser Karl Rove, whose role in the firings has been under scrutiny by congressional Democrats.

Democrats have charged that Rove and other officials may have used the private accounts, set up through the Republican National Committee, in an effort to avoid normal review. Under federal law, the White House is required to maintain records, including e-mails, involving presidential decision-making and deliberations. White House aides' use of their political e-mail accounts to discuss the prosecutor firings has also fanned Democratic accusations that the actions were politically motivated
...
Briefing reporters yesterday about an initial review of the private e-mail system, White House spokesman Scott Stanzel declined to discuss whether the political aides were driven by a desire to conduct business outside of potential review. "I can't speak to people's individual e-mail practices," he said.

Stanzel conceded that the White House had done a poor job of instructing staff members how to save politically oriented e-mail and said that it has developed new guidance for the more than 20 staffers who have official as well as political e-mail addresses. He also said that the White House is trying to recover the lost e-mails.

"The White House has not at this point done a good enough job at overseeing the practices of staff with political e-mail accounts," Stanzel said. "Some officials' e-mails have potentially been lost and that is a mistake that the White House is aggressively working to fix."
...

source: Washington Post

Waxman advises RNC not to destroy any e-mails
Michael Roston - Monday March 26, 2007
Waxman advises RNC not to destroy any e-mails

Pointing to e-mails between Bush administration officials and convicted lobbyist Jack Abramoff that used private e-mail addresses, the Chairman of the House Oversight and Government Reform Committee ordered the Republican National Committee and the Bush/Cheney 2004 campaign to preserve all e-mail records and to ensure that they aren't purged or destroyed.

"White House officials have used nongovernmental e-mail accounts, including those maintained by the RNC [other non-governmental accounts besides the RNC's?], to conduct official White House business," Rep. Henry Waxman (D-CA) wrote in letters delivered today, copies of which were sent to RAW STORY. "The Committee has questions about who has access to these e-mail records and how the RNC protects them from destruction and tampering."
...
Waxman also noted that according to the terms of the Presidential Records Act of 1978, "Such e-mails written in the conduct of White House business would appear to be governmental records subject to preservation and eventual public disclosure."
...