When you do not know who will be receiving the letter.
The former.
It means to put from or sincerely your name
A colon is used as in the following example:To Whom It May Concern:
It is a single word 'whomsoever', not two words 'whom' and 'soever' as it is mentioned by you. The sentence 'To whomsoever it may concern' means the matter which would follow this sentence may have...
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