One of National Australia Bank's longest-serving executives, Steve Lambert, has resigned after failing to meet standards of behaviour expected by his employer.A NAB spokesman confirmed a report in The Australian newspaper on Monday that revealed Lambert's departure from the bank, but refused to give specific details of the events that triggered the decision.?"In speaking with NAB, Steve acknowledged he had fallen short of the standards of behaviour expected at NAB and that it would be the right thing to leave the bank," the spokesperson said.Since joining the Melbourne-based bank from Westpac in 1998, Lambert has been one of the country's most innovative capital markets executives and is widely recognised for his pioneering work in the development of green bonds in the Australian market.In December 2014, Lambert managed the issue of the first Australian climate bond certified under the global climate bonds standard.The proceeds of the issue are used by NAB to fund lending to borrowers in the renewable energy sector, including wind farm operators and generators of solar power.Lambert also had a role in the formation in October 2011 of loan fund manager, Metrics Credit Partners, which resulted in NAB being able to market domestic loans to investors at a time when institutional investors had abandoned securitised assets.Lambert remained involved in MCP as an alternative director nominated by the bank after it was spun out of the group in 2012.In recent years Lambert's influence extended well beyond his role as executive general manager of capital financing at the bank.He has been a director of the not-for-profit Women in Banking and Finance, a forum that aims to enhance opportunities for women to advance in the industry.Until this week Lambert was a member of the global advisory board to the United Nations' Environment Finance Initiative.