Leading scorer Lee Trundle opened the scoring for the home side in the 55th minute with a great close-range strike.
The Shrimpers bounced straight back and Wayne Gray took advantage of slack marking by the home defence to level the scores with a snap-shot into the corner - meaning the Shrimpers closed the gap between them and third-placed Scunthorpe to three points.
Bottom club Cambridge earned a point from their trip to play-off hopefuls Northampton.
A second-half double strike by Cambridge striker Daniel Chillingworth almost earned his side their first away win of the campaign.
He cancelled out David Rowson's early goal for the Cobblers and again capitalised on schoolboy defending by the home side to make it 2-1 in the 59th minute.
But Cambridge's hopes of securing that elusive first win on the road were dashed when Scott McGleish notched his 15th goal of the season with nine minutes left.