preposition
BAD : Regarding to your letter of 22 November, I have referred your query to our accounts department.
GOOD : Regarding your letter of 22 November, I have referred your query to our accounts department.
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regarding (WITHOUT to ): 'If you have any questions regarding any of our services, please feel free to contact me.'
See also REFERENCE (↑ reference ), REGARD 1 (↑ regard )