- Machine Shop & Tool Repair Service, Southold
- Machinery, Supplies & Equipment - Construction & Contractor Related, Southold
- Major Appliances, Southold - Household & Kitchen
- Management Consultants & Services, Southold
- Manufacturing, Southold
- Marinas & Marina Services, Southold
- Marine Contractors & Designers, Southold
- Marine Products And Services, Southold
- Marine Repair & Service Stations, Southold
- Marine Services, Southold
- Marine Supplies & Equipment, Southold - Boat Supplies
- Marine Supplies & Equipment Repair, Southold - Boat Supplies & Equipment
- Marriage & Family Counselors, Southold
- Martial Arts & Self Defense Schools, Southold
- Masonry Contractors, Southold - Stone Mason, Brick Mason - Install & Repair
- Massage Therapists & Professional, Southold
- Massage Therapy, Southold
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- Meat Market, Wholesale & Products, Southold
- Medical Supplies & Equipment, Southold
- Membership Sports & Recreation Clubs, Southold
- Mental Health Specialists, Southold - Psychologist, Psychiatrist, Psychotherapist, etc
- Metaphysical Churches, Southold
- Miscellaneous Government, Southold
- Miscellaneous Personal Services, Southold
- Miscellaneous Professional, Southold
- Mobile Coffee & Food Service, Southold
- Mobile Coffee & Food Service, Southold
- Mobile Restaurants, Southold
- Motor Vehicle, Southold
- Motor Vehicle Rental & Leasing, Southold
- Moving Truck Rental , Southold
- Mulch, Peat & Moss Products, Southold
- Museums, Southold
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