The Canadian Union of Postal Workers plans to take Canada Post to court to challenge the federal government's legislation forcing its members back to work.

The back-to-work legislation was passed into law Sunday, and mail began to be delivered Tuesday.

The union says it won't defy the law, and mail delivery will continue — but it will challenge the legislation in court, arguing in part that it discriminates against new employees.

The union's national executive made the decision at a meeting in Ottawa on Tuesday to decide on action.

The Harper government's legislation ending the strike came after 12 days of rotating strikes, a 13-day lockout, and 58 hours of filibuster in the House of Commons as the NDP Opposition attempted to delay a vote on the bill.

The union is also considering lodging a complaint with the Human Rights Commission, claiming discrimination against newer employees, who will not have the same pension benefits as older ones.

The back-to-work legislation forces postal workers to accept wages that are less than Canada Post's last offer.

On non-wage issues, it imposes a form of winner-take-all arbitration. CUPW and the corporation will each make a final offer, one of which will be chosen by the arbitrator.

Salary issues aren't included in the arbitration process.

Meanwhile, mail delivery resumed Tuesday as postal workers made a small dent in the millions of letters, bills and flyers that went undelivered during the strikes and lockout.

Letter carriers were back on their routes again, two days after Parliament ordered the 48,000 postal employees to return to work.

Some carriers complained they were not allowed to work overtime to restore service faster. Canada Post, however, said it’s too early to know if overtime is needed.

The Crown corporation repeated its plea for patience as it tackles a big backlog of mail either piled up inside regional plants or waiting to get in.

With files from The Canadian Press