New York fell a game behind Milwaukee in the National League wild-card chase and also dropped two games back of Philadelphia in the NL East title hunt.
The Mets can do no better than tie the Phillies for the division crown. If both teams have clinched playoff spots at the end of the regular season New York would win the division title in a tiebreaker because the Mets won the season series with the Phillies.
If New York and Philadelphia wind up tied for the division crown and Milwaukee is the wild-card team, the Mets and Phillies would meet in a one-game playoff for the NL East championship.
Volstad (6-4) gave up five hits and struck out four in a 101-pitch effort. He left the game with a 3-1 lead and the Marlins expanded their advantage with three late runs -- Hanley Ramirez scoring one of them and driving in another.
Mike Pelfrey (13-11) took the loss and is winless in his last six starts. He surrendered eight hits and two runs over six innings.