It looks like the table alias names SND and M are confusing you so remove them.
If this is now your tables

TABLE users
userid   firstname    lastname
1         JOHN         DEO
2         JANE         DEO

TABLE msg
msg_id    msg_from     msg_to      received   age  city  country
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The correct SQL (without alias names) is

SELECT users.userid, users.firstname, users.lastname, msg.age, msg.cit
+y, msg.country FROM msg
JOIN users ON users.userid = msg.msg_from
WHERE msg.msg_to = ? 
  AND msg.received IS NULL
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