A mailbox provider, sometimes called email service provider, is a department or organization that provides email hosting. It provides email servers to send, receive, accept, and store email for other organizations and/or end users, on their behalf and upon their explicit mandate.
Ambiguity of the term 
Especially in the U.S., the term email service provider refers to a company that sends bulk email for marketing or similar purposes. The term mailbox provider was proposed because it is not ambiguous. The title of this article sticks to that proposal. However, various writings even from supposedly knowledgeable sources use the ambiguous term instead.
Types of mailbox providers 
There are various kinds of email providers. There are paid and free ones, possibly sustained by advertizing. Some allow anonymous users, whereby a single user can get multiple, apparently unrelated accounts. Some require full identification credentials; for example, a company may provide email accounts to full time staff only. Often, companies, universities, organizations, groups, and individuals that manage their mail servers themselves adopt naming conventions that make it straightforward to identify who is the owner of a given email address. Besides control of the local names, insourcing may provide for data confidentiality, network traffic optimization, and fun.
Mailbox providers typically accomplish their task by implementing Simple Mail Transfer Protocol (SMTP) and possibly providing access to messages through Internet Message Access Protocol (IMAP), the Post Office Protocol, Webmail, or a proprietary protocol. Parts of the task can still be outsourced, for example virus and spam filtering of incoming mail, or authentication of outgoing mail.
ISP-based email 
Many mailbox providers are also access providers. Not the core product, their email services could lack some interesting features, such as IMAP, Transport Layer Security, or SMTP Authentication —in fact, an ISP can do without the latter, as it can recognize its clients by the IP addresses it assigns them.
Free mail providers 
Launched in the 90s, AOL Mail, Pobox, Hotmail, Yahoo! Mail, Lycos, Mail.com, and Gmail in 2004 are among the early providers of free email accounts. They attract users fast because they are free and can self-advertize their service on every message. According to Jurvetson, Hotmail grew from zero to 12 million users in 18 months. That was before it was bought my Microsoft.
Premium email services 
These are the paid equivalent of free mail providers. That is, a better alternative to ISP-based email. Much less popular than free mail, they target a niche of users.
Vanity email 
It is also possible to run a shim service, providing no access but just forwarding all messages to another account, which does not lend itself to direct use, for example because it is temporary or just less appealing.
Role as identifier 
A mailbox provider is the administrator of the registered domain name that forms the domain-part of its email addresses. As such, it controls the MX records that specify which hosts will receive email destined to those addresses. The operators of those hosts define the meaning of the local-part of an address by associating it to a mailbox, which in turn can be associated to a user. The mailbox provider also specifies how users can read their mail, possibly creating SRV records to ease email client configuration, or giving detailed instructions.
Email addresses are convenient tokens for identifying people, even at web sites unrelated to email. In fact, they are unique, and allow password reminders to be sent at will.
It has to be noted, for the bureaucracy-oriented, that this role is based on IETF standards. Unlike X.400, there is no formal undertaking beyond domain name registration. The notion of Administration Management Domain (ADMD) is derived afterwards, from empirical evidence.
See also 
- J.D. Falk (8 September 2009). "ESP". WG discussion. IETF. Retrieved 14 April 2013.
- "Where to find your e‑mail account information". Microsoft. Retrieved 14 April 2013. "Your e‑mail service provider should provide you with the information you need to sign in to your e‑mail account."
- Murray Kucherawy, ed. (June 2012). Creation and Use of Email Feedback Reports: An Applicability Statement for the Abuse Reporting Format (ARF). IETF. RFC 6650. https://tools.ietf.org/html/rfc6650. Retrieved 28 June 2012. ""Mailbox Provider" refers to an organization that accepts, stores, and offers access to [RFC5322] messages ("email messages") for end users. Such an organization has typically implemented SMTP [RFC5321] and might provide access to messages through IMAP [RFC3501], the Post Office Protocol (POP) [RFC1939], a proprietary interface designed for HTTP [RFC2616], or a proprietary protocol."
- J.D. Falk, ed. (November 2011). Complaint Feedback Loop Operational Recommendations. IETF. RFC 6449. https://tools.ietf.org/html/rfc6449. Retrieved 28 June 2012.
- Jurij Leskovec (2008), Dynamics of Large Networks, ProQuest, ISBN 9780549957959
- John Klensin (October 2008). Simple Mail Transfer Protocol. IETF. RFC 5321. https://tools.ietf.org/html/rfc5321. Retrieved 14 April 2013.
- Dave Crocker (July 2009). "Administrative Actors". Internet Mail Architecture. IETF. sec. 2.3. RFC 5598. https://tools.ietf.org/html/rfc5598#section-2.3. Retrieved 18 April 2013.