SELECT NULL AS msg_id, NULL AS total, NULL AS found
    FROM dual
     WHERE (
      @total :=0
       OR
      @found :=0
    )
    UNION
      SELECT msg_id, @total AS total, @found :=1 AS found
        FROM messages_queue
      WHERE (
        @total := @total + rcpts_count
      )
    AND @total <1000
    UNION
      SELECT msg_id, rcpts_count AS total, 0 AS found
        FROM messages_queue
      WHERE IF( @found =0, @found :=1, 0 )

-- sample table
create table x
    (col1 int identity(1, 1)
    ,col2 varchar(50))

-- sample data
insert into x (col2) values
     (null)
    ,(null)
    ,(null)
    ,(null)
    ,(null)

-- update from select
update x
    set x.col2 = 'even'
from x as [t2]
where
    col1 = t2.col1
    and t2.col1 % 2 = 0

-- show results
select * from x

-- clean up
drop table x

UPDATE message_queue mq
INNER JOIN (
   SELECT NULL AS msg_id, NULL AS total, NULL AS found
    FROM dual
     WHERE (
      @total :=0
       OR
      @found :=0
    )
    UNION
      SELECT msg_id, @total AS total, @found :=1 AS found
        FROM messages_queue
      WHERE (
        @total := @total + rcpts_count
      )
      AND @total <1000
    UNION
      SELECT msg_id, rcpts_count AS total, 0 AS found
      FROM messages_queue
      WHERE IF( @found =0, @found :=1, 0 )
) msgs ON msgs.msg_id = mq.msg_id
SET mq.fet_id = 12345;