I'm getting a frustrating error in one of my SQL Server 2008 queries. It parses fine, but crashes when I try to execute. The error I get is the following:

Msg 8120, Level 16, State 1, Line 4
Column 'customertraffic_return.company' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

SELECT *
FROM   (SELECT ctr.sp_id                     AS spid,
           Substring(ctr.company, 1, 20) AS company,
           cci.email_address             AS tech_email,
           CASE
             WHEN rating IS NULL THEN 'unknown'
             ELSE rating
           END                           AS rating
    FROM   customer_contactinfo cci
           INNER JOIN customertraffic_return ctr
             ON ctr.sp_id = cci.sp_id
    WHERE  cci.email_address <> ''
           AND cci.email_address NOT LIKE '%hotmail%'
           AND cci.email_address IS NOT NULL
           AND ( region LIKE 'Europe%'
                  OR region LIKE 'Asia%' )
           AND SERVICE IN ( '1', '2' )
           AND ( rating IN ( 'Premiere', 'Standard', 'unknown' )
                  OR rating IS NULL )
           AND msgcount >= 5000
    GROUP  BY ctr.sp_id,
              cci.email_address) AS a
WHERE  spid NOT IN (SELECT spid
                FROM   customer_exclude)
GROUP  BY spid,
      tech_email