About the only aspect of mail art that one can state with any degree of certainty is that it is entirely dependent upon the international postal system for its existence. More broadly speaking it's roots are in the opposition to the establishment art world of the 1960's, and as such, it adopted much of this ethos of resistance, coupled with the expanded concepts of art and artmaking circulating at this time.
However, what was sent through this system and the manner in which it acted as a catalyst for innumerable and varied projects defies categorization in any traditional terms. Quite self-consciously set-up to operate outside of the artworld establishment and thereby circumventing the commodification inherent in that system, it has to a large extent succeeded in this important respect. How this has been achieved was to take the money out of the artmaking paradigm, and to replace it with a paradigm based upon reciprocity and exchange.
One other important element in the construction of this new paradigm was the attempt at collapsing the dichotomy between art and life, and subsequently between artist and non-artist. This aspect of all-inclusiveness encouraged an interrogation of the arena of 'social space,' and did much to maintain the vitality of mail art, allowing it to expand, particularly in its early period 1970-75. (1)
Ray Johnson's correspondence activities, formally associated with his New York Correspondence School (NYCS) in 1962 until its demise in 1973, have arguably been one of the more influential factors in the spread of mail art activity and the exponential growth of this network from the late 60s through the 70s. Lending credence to this activity, popularizing it further, and providing an impetus for further exhibitions, was the exhibition of the NYCS at the Whitney Museum in 1970, organized by Ray Johnson and Marcia Tucker.
The period at the end of the 60s and beginning of the 70s marks an important point in the development of mail art from a private form of communication between groups such as Fluxus, the French New Realists, the NYCS and other assorted groupings into a more public and open network of correspondents. One very influential and significant means by which this expansion occurred was through the publication of international address lists collated from varied sources. In 1972, Ken Friedman in conjunction with Image Bank in Canada, and others, published a list containing over 1400 names and addresses. Other published lists of this period would include Klaus Groh's International Artists Cooperation Information Sheet (Germany, 1971-78), and Giancarlo Politi's yearly Art Diary, which listed international information on individuals and institutions.
Another means by which the mail art network expanded was directly related to the increasing number of mail art shows organized during this decade. The catalogues and documentation for these shows, which included participants' names and addresses, were another way in which contact was promoted. (2)
Parallel to the expansion of the mail art network, the early 70s also saw the beginnings of a number of artists' publications growing out of the emerging mail art network (as distinct from artists' publications arising out of tangential artists' groupings.) In these early publications there is a convergence of work by artists who's concerns overlapped with those of the mail art network, but who were nevertheless rooted in parallel concerns (Fluxus would be a good example).
An early example of a magazine where this convergence took place, and which metamorphosed with the mail art network was the New York Correspondence School Weekly Breeder. Started by Ken Friedman from Fluxus West, it was originally a single-sheet weekly publication that was intended as a means of maintaining contact with other artists. Friedman passed the editorship to Stu Horn in Philadelphia for a couple of months, and then in turn it was passed onto the Bay Area Dadaists (Bill Gaglione & Tom Mancusi) in San Francisco who published it until 1974.
In the last issue of the New York Correspondence School Weekly Breeder the Bay Area Dadaists stated that it represented, "...the realization of the modern dadazine and was influential in inspiring the current 'zine scene..." (3,4). The introduction of the word 'zine,' (although not the first time it was used in the early 70s), expresses a particular attitude in regard to the magazines that would be produced from this period onwards. Michael Crane summarizes the characteristics of the zine scene in the 70s:

Hundreds of 'zines have appeared in the last decade, some more long-lasting or influential than others. In them, verbal and visual elements are fused, associated, or juxtaposed through collage, postcards, and rubber stamps. Concrete or visual poetries abound; words replace, displace, or fragmentize images in most works. Unlike dada or futurism, most mail art does not spring from literary concerns. Like dada or futurism, however, political concerns are voiced in geographic areas of the world where the socio-political climates are the most volatile. Cliches, humor, and puns, whether friendly, frivolous, or satirical appear in 'zines, coinciding with traditions in artmaking begun in the earlier part of this century. Ambiguity, self-reference, contradiction, and loose or personal associations also earmark these publications. Some include all types of work, while theme 'zines have a common format or idea. Most 'zines do not appear to be serious works but this is often the point. Others are designed and produced to resemble commercial magazines. (5)

What becomes evident from Crane's description of zines in the 70s, is that they can be characterized as much by their irreverence and eclecticism as they can be by their specific content.
Although I have been using the term 'network' in reference to the 'mail art network' up to this point, it should be noted that it was really only during this early part of the 1970s when a flurry of mail art inspired publications started appearing that the participants became aware that they were indeed part of a 'network.' (6,7) This realization of a geographically decentralized network was in great part due to these publications themselves, for not only did they put people in contact with each other (the inclusion of contributors addresses also promoting this contact), but they also acted as the bridges that fostered communication, interaction and facilitated the dissemination of artworks and announcements for projects between these disparate groupings of national and international artists.
One, among a handful of magazines during the early 70s that played a seminal role in the mail art network was File magazine (1972-1989), initiated principally by A.A. Bronson in Canada and founded as a parody of Life magazine, it was, in Anna Banana's opinion:

...the most influential of the Mail Art 'Zines, in that it popularized the whole process by publishing and widely distributing an extended Image Bank Request List, along with news and photos of artists from "the eternal network." It was more effective than any other of the small publications individuals were putting out, because it was a full size magazine, with space to give news of the activities of many persons active in the Mail Art scene. (8)

In later years General Idea, as the editors of File came to be known, would describe the founding of File as, "...a networking publication...," and further in this article (referring to issues after Fall 1975 when they discontinued listing artists addresses and image requests, thereby effectively breaking with the mail art network), they say, "But instead of servicing and addressing our peers in the Eternal Network we came to be talking demographics." (9) With its glossy covers, efficient distribution and relatively large printing runs from between 3000-5000, File did much to popularize the mail art network, such that by 1973 many of the first generation mail artists quit the network in disgust at what they perceived to be the low quality of the work that subsequently inundated their mail boxes.
One of the specific criticisms leveled at the network by those who quit during this period was the appearance of a large volume of work which made use of xerography ("quick-kopy krap" (10)). Ideally suited because of its accessibility and affordability, to mailing multiple copies to a host of destinations, these early years between 1970-73 sees xerography become an integral part of the network. This is also reflected in the larger number of mail art publications that would rely on xerography for the production of small, economical and quickly assembled publications.
While the early years of the 1970s saw a steady rise in the number of publications, the figures rise dramatically from 1975 onwards. This expansion can be seen as the direct result of the consolidation of the network through the role played by the publications from this early period, coupled with the growing number of assembling magazines and the steady rise in the number of mail art exhibitions. All these artists' publications were in varying degrees connected to the network and published work acquired from this source, or listed projects and publications emanating from the network.
For the moment general descriptions will have to suffice for the large variety of publications, and the following labels are derived from their predominant concerns: a) many mail artists started their own personal movements, organizations, or fictitious companies and published magazines that reflected and promoted these concerns; b) specific issues of publications that were concerned with a particular theme; c) neo-dada publications; d) intermedia publications; e) conceptually oriented publications; f) performance and live art publications; g) publications that were primarily concerned with disseminating information about network projects and shows; h) graphics and collage publications; i) concrete, visual and language poetry publications; j) rubber stamp publications: k) neo-fluxus publications.
The following points summarize the different roles that these publications played in this period:

• They were vital in creating national and international links between disparate groupings of individual artists.
  
• They contributed to a large degree in the expansion and consolidation of the network, particularly in the early years during 1970-75.
  
• They facilitated the transmission of new and expanded ideas about art and artmaking, leading to an increased awareness of the social role that art and the artist played within what had become a mass mediated culture.
  
• They provided a crucial framework in creating and sustaining an independent and underground network of like-minded participants.