When you’re purchasing an Office 365 account, you need to decide how many users you want to buy. You'll need a separate user for each individual person who will have their own, personal mailbox. However, when it comes to other addresses like sales@yourdomain.com or support@yourdomain.com, there are several options for creating free addresses. These give you flexibility without needing to add additional users.
Aliases are a good way to make a small operation look a bit bigger. An alias is a completely different email address that sends all mail to the original mailbox. If molly@mollysbarkandwine.com sets up an alias, jobs@mollysbarkandwine, all mail sent to either molly@ or jobs@ goes to the molly@ mailbox. Molly has 2 email addresses, but has only purchased 1 user. Read more about aliases.
Shared mailboxes are a good way to provide a generic email address that many people can access. For example, Molly sets up questions@mollysbarkandwine.com as a shared mailbox. She shares it with herself (molly@mollysbarkandwine.com) and her partner (joe@mollysbarkandwine.com). They can both read and reply to mail sent to questions@ but Molly only has to buy 2 users, one for herself and one for Joe. Read more about shared mailboxes.
Distribution groups are another way to extend your address count. Typically, a distribution group is used to communicate with a group of people internally, but you can also have external contacts in your distribution group list. Molly could set up a distribution group for all her servers, for her vendors, and her VIP customers, all for free. She wouldn’t need to buy users for any of those distribution group addresses. Read more about distribution groups.
Shared contacts are people outside of your domain who you want to include in a distribution group. These accounts are also free; you do not need to buy an extra user to create a shared contact.