Here is what I found in the Mojang Commercial Usage Guidelines:

SERVERS AND HOSTING

With hosting servers, we want to enable the community to make money by creating, hosting, and maintaining servers for Minecraft, so you may do this subject to these Guidelines. By "server," we mean a single connecting address or IP number.

Therefore, if you comply with and follow the Brand and Asset Usage Guidelines and Naming Guidelines above, YOU MAY:

• charge for access to your server, including a server which hosts your Mods, BUT only if:
  • you make a charge per person that is the same for everyone;
  • you give everyone you charge access to all the Mods that you choose to have on your server (except only in respect of genuine admin tools / admin Mods which should be reserved to administrators);
  • you only give access to your server to users who have a genuine paid for version of Minecraft;
  • you own or control the server and continue to do so for the whole time that you charge for access to it - so, if you sell the actual servers or server space with Mods pre-installed on them, you must ensure that the person buying the server is aware that she must follow these rules. Essentially you can’t get around the above by setting up servers with Mods on and then selling those set-ups
• ask for donations (as opposed to direct charges) IF you do not offer the individual donor something in exchange that only he or she can use. You may offer server wide rewards if donation goals are achieved though.
• sell entitlements that affect gameplay provided that they do not adversely or negatively another player’s experience and provided they do not give a competitive gameplay advantage. A competitive gameplay advantage is something that, given identical skill levels, time investment, and circumstances, can cause one player or group to perform better than another.
  • Be aware that certain games or game modes can be designed as non-competitive (such as a player-versus-environment (PvE) game) and become competitive by adding gameplay elements such as a leaderboard.
• sell cosmetic entitlements, except for "Capes," which we define as anything that attempts to visually emulate the feature of a Minecraft player Cape.
• provide in-game display and video advertising, sponsorships, or product placement for third-parties IF (i) they don’t degrade or interfere with gameplay or give a user any competitive gameplay advantage over anyone else on the server and (ii) you follow the rules outlined in “Constructed Promotions in Minecraft” below.
• use and sell in-game virtual currencies IF (i) they have no real-world value and that cannot be cashed out, used, or transferred across free or paid servers, or into any other currency with real-world value; (ii) you don't give the impression that it comes from or is associated with Mojang, (iii) they don't look like or have similar names to Minecoins or any other official virtual currency, and (iv) anything you sell with the currency follows our monetization guidelines.

In all of these cases above YOU MUST also ensure that:

• all servers, entitlements, and advertising are suitable for children and minors (i.e., no gambling, pornography, etc.) and don’t harm the Brand.
• you don’t pretend to be Mojang or Microsoft or claim to have any association with Mojang or Microsoft (for example, when selling product placement opportunities);
• you are explicit about who to contact about the server, who the operator is, and that it is NOT Mojang or Microsoft, NOT associated with Mojang or Microsoft and NOT supported by Mojang or Microsoft;
• Neither Mojang nor Microsoft has any liability for anything on the server or that happens on it, including any advertising, purchases, or donations;
• you provide any user who gives you real-world currency an online payment / purchase history for purchases made with that currency;
• you are transparent on informing users with regard to all the content and pricing applicable to your server before a person joins, signs up, or logs in;
• you do not host or distribute malware, spyware, or engage in illegal activity through your server;
• if you hire any social influencers for a promotion (e.g., YouTube, Mixer, Instagram, Twitch) you will require the influencer to disclose their relationship with you and to follow the disclosure guidelines published by the U.S. Federal Trade Commission at:
  https://www.ftc.gov/tips-advice/business-center/guidance/disclosures-101-social-media-influencers;
• you comply with all other laws and requirements; and
• you comply with any other additional requirements applicable under these Guidelines.

We want Minecraft to be a place where players can have fun and a trusted place to explore, build, and invest their time and money. Please use your common sense and discretion in creating a safe, fun, and trusted environment for all players. We reserve the right to adjudicate and interpret whether the way you monetize is done in a manner consistent with our Brand. In short, don’t do evil things to players.