I am updating my resume. I am trying to say as part of my job description that I did email corresponding with customers and "other companies", but Im trying to be more professional with the term "other companies". As an example the people you work with are your "co-workers". People you go to school with are your "peers" How would you classify or name the people/ companies that you corresponded with?
As the comments suggest, your options for describing these other companies depends on their relationship to your employer. This isn't just a usage question, i.e., it's not just the correct tone and register for a resume. It speaks to the skills and experience you demonstrated by working with those companies.
Some broad terms:
Clients: a company can have consumer clients as well as business clients. Sometimes a business client is called an enterprise client. If the other company paid your employer for a product or a service, that company is a client. If you are in sales or marketing, you may deal with current and potential clients
Service provider or supplier: if your employer purchases services from another company, that company is a service provider. The term "service" applies broadly. A service provider can be an accounting firm, plumber, financial services firm, utility, consulting firm, etc. If your employer purchases goods from another company, that company is a supplier. This applies whether those goods are processed and/or resold by your company (raw materials, wholesale products) or if those goods are consumed by your company in the course of doing business (computers, toilet paper). As with clients, you may deal with potential service providers and potential suppliers.
Partners and competitors: Companies work with partners to provide goods or services to a client or consume goods or services from a supplier or service provider. Companies compete with competitors for the same. These terms are not mutually exclusive. A company may be part of a trade association with a competitor, and may work together to lobby or promote awareness, but still compete with each other for clients.
Each of these terms describes a relationship rather than a specific entity. For example, a wholesaler may sell their product to a retail store for resale and also sell their product directly to consumers on their website. Here the wholesaler is a supplier and a competitor of the retail store. The supplier/client relationship could also be described as a partnership, making the two companies partners and competitors. Use a term that describes the relationship you were involved in.