I know that, in the mid-90s when I was working at AOL as their Sr. Internet Mail Administrator, we had accounts on the other major providers systems -- Prodigy,

I know that, in the mid-90s when I was working at AOL as their Sr. Internet Mail Administrator, we had accounts on the other major providers systems -- Prodigy, CompuServe, etc.... We also knew who our counterparts were at those other companies, and in fact it was Ed Raven at Prodigy who lead me to interview with Jay Levitt at AOL, and I got the job. At different stages, it might make sense to be more careful with how you let people sign up for your service, and who you let sign up. But in our case, it was beneficial for everyone involved to know who their counterparts were and to have accounts on their systems. Among other things, this let us build up tests for our systems that could quickly determine if there were major delays in getting e-mail from our systems to theirs, and if the delay was likely to be at their end or ours. And if it was at their end, it was to our benefit to notify them of this fact, because it was also impacting our own customers. But I can only speak for my personal experience.