I know of no evidence to suggest that the postal clerks died in the mail room. Indeed, it's hard to see how they could have been trapped by rising water in that location. At least one of them spoke to stewardess
Violet Jessop on the boat deck, some time after they had given up their losing battle to save the mails and had done the sensible thing and gone topside.
Mrs Snape did say that she didn't expect to see her passengers again, but that could have been related to the commonly-held belief that staying on the ship was safer than being lowered into the ocean in a small boat. According to comments by other stewardesses, she (like the 3rd Class matron) eventually went down to her room and couldn't be persuaded to come out.
Joao, What is your source for the story of Mrs Snape giving up her lifeboat place? There are a number of such accounts by survivors, but unfortunately the passengers were rarely able to identify crew members by name. Minnie Coutts, for instance, told of a crew member (most likely a 3rd Class steward) who guided her to the boat deck and gave her his own lifebelt with the words "If the ship goes down you'll remember me". As you say, no doubt there were many other selfless acts which were never placed on record.
There is a good example of quiet devotion to duty in
Violet Jessop's memory of "good, faithful old Stanley", the long-serving steward who assured her that the ship was sinking and urged her to hurry on deck to save her life. But when she did climb the stairs to the boat deck Stanley didn't follow. He seemed rather to have accepted his fate with quiet resignation: "Halfway up, I looked down and waved to Stan. He was standing with his arms clasped behind him in the corner where he usually kept his evening watch. He suddenly looked very tired".
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