The review comes after the DM incumbent on the business, the Bristol-based integrated agency Flourish, parted company with the organisation after three years.
The Samaritans has now shortlisted 3 agencies to pitch for the account, with an appointment set to be made in April.
Flourish was first appointed to the Samaritans' DM business in 2007, without a pitch, and has since created a number of campaigns for the charity, including a mailpack that took the form of an advent calendar to highlight the number of calls that it receives around the Christmas period.
Before Flourish's appointment, the Samaritans DM account was handled by Kitcatt Nohr Alexander Shaw, which won the business in 2004 after a four-way pitch.
The agency created a high-profile mailpack that explained how suicidal people have gone on to live a prosperous life after making a call to the Samaritans.
The campaign, which used the strapline "With Samaritans, life goes on", boosted the average donation fourfold and picked up a commendation at the 2007 Campaign Direct Awards.
Lunar BBDO was the most recent incumbent on the charity's TV advertising business, and Mindshare is the incumbent on the media planning and buying account.