A man should be given his title if he possess one. Otherwise he must be addressed as "Mr." or "Esq."
Titles of those holding public office, of physicians, of the clergy, and of professors, are generally abbreviated on the envelope except in formal letters.
It is rather customary to address social letters to "Edward Beech, Esq.," business letters to "Mr. Edward Beech," and a tradesman's letter to "Peter Moore." A servant is addressed as "William White."
The idea has arisen, and it would seem erroneous, that if the man addressed had also "Sr." or "Jr." attached, the title "Mr." or "Esq." should not be used. There is neither rhyme nor reason for this, as "Sr." and "Jr." are certainly not titles and using "Mr." or "Esq." would not be a duplication. So the proper mode of address would be
Mr. John Evans, Jr.
or
John Evans, Jr., Esq.
The "Sr." is not always necessary as it may be understood.
Business envelopes should have the address of the writer printed in the upper left-hand corner as a return address. This space should not be used for advertising.
In addressing children's letters, it should be remembered that a letter to a girl child is addressed to "Miss Jane Green," regardless of the age of the child. But a little boy should be addressed as "Master Joseph Green."