- Machine Shop, South Portland
- Machine Shop & Tool Repair Service, South Portland
- Machinery, Supplies & Equipment - Business Production Related, South Portland
- Machinery, Supplies & Equipment - Business Supplies Related, South Portland
- Machinery, Supplies & Equipment - Construction & Contractor Related, South Portland
- Mailbox Rental Services, South Portland
- Mailing List Service, Brokers & Compilers, South Portland
- Mailing, Delivery & Courier Service, South Portland
- Maintenance, Cleaning & Sanitation, South Portland
- Major Appliances, South Portland - Household & Kitchen
- Malls & Shopping Centers, South Portland
- Management Consultants & Services, South Portland
- Manicurists & Pedicurists, South Portland
- Manufacturers Agents & Representatives, South Portland
- Manufacturers Representative, South Portland - Reps, Independent Sales Representative, Sales Agent, etc
- Manufacturing, South Portland
- Manufacturing & Industrial Consultants, South Portland
- Manufacturing Products, South Portland
- Marinas & Marina Services, South Portland
- Marine Supplies & Equipment, South Portland - Boat Supplies
- Market Research & Analysis , South Portland - Market Analysis, Business & Economic Research
- Marketing Schools, South Portland
- Marriage & Family Counselors, South Portland
- Martial Arts & Self Defense Schools, South Portland
- Masonry Contractors, South Portland - Stone Mason, Brick Mason - Install & Repair
- Masonry Materials & Supplies, South Portland
- Massage Therapists & Professional, South Portland
- Massage Therapy, South Portland
- Material Handling & Heavy Machinery & Equipment, South Portland
- Material Handling Consultants, South Portland
- Material Handling Equipment, South Portland - Conveyor Belt, Pallet Jack, Forklift, etc
- Maternity Clothes & Clothing Stores, South Portland - Maternity Dress, Gown, Swimwear, Underwear & More
- Mechanical Engineer, South Portland - Mechanical Engineering Service
- Medical & Dental Assistant Schools, South Portland
- Medical Identification Jewelry, South Portland
- Medical Supplies & Equipment, South Portland
- Men's & Boys Clothing & Accessories Stores, South Portland
- Men's Clothing Stores, South Portland - Clothes & Accessories
- Men's Shoe Stores, South Portland
- Mental Health Specialists, South Portland - Psychologist, Psychiatrist, Psychotherapist, etc
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- Merchandise Sales Events, South Portland
- Merchandising & Pricing Equipment, South Portland
- Messenger Service, South Portland - Message Receiving & Forwarding Service
- Metal Lockers, South Portland
- Metal Products, South Portland
- Methodist Church, South Portland
- Mexican Restaurants, South Portland
- Middle Eastern Restaurants, South Portland
- Millwork Contractors, South Portland
- Mini & Self Storage, South Portland
- Mining, South Portland
- Mining Consultants, South Portland
- Mining Contractors, South Portland - Mining Companies, Contractors & Brokers
- Mining, Crude-Oil Production, South Portland - Mining Companies
- Miscellaneous Business Services, South Portland
- Miscellaneous Government, South Portland
- Miscellaneous Industry, South Portland
- Miscellaneous Manufacturing, South Portland
- Miscellaneous Personal Services, South Portland
- Miscellaneous Photographers, South Portland
- Miscellaneous Professional, South Portland
- Money Transfer Service, South Portland
- Monuments, Tombstones, Markers & Memorials, South Portland
- Mortgage Bankers & Correspondents, South Portland
- Mortgage Companies, South Portland
- Mortgages Commercial & Industrial, South Portland
- Motion Picture Pre & Post Production Services, South Portland
- Motor Vehicle, South Portland
- Motor Vehicle Rental & Leasing, South Portland
- Movie Theaters, South Portland
- Moving & Storage Supplies & Equipment, South Portland
- Moving Companies, South Portland
- Moving Truck Rental , South Portland
- Muffler Shop & Exhaust Systems, South Portland
- Museums, South Portland
- Music, South Portland
- Musical Instruments & Equipment Stores, South Portland
- Musical Instruments - Violins, South Portland
- Musical Instruments Repair, Sales & Service, South Portland
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