Inverse Network Technologies recently completed a study of electronic mail performance. The company determined that between April and September, 1997, the number of messages which were delivered within five minutes dropped from 92 percent to 81 percent. Tracking was done every two weeks during the six-month period. There was a steady month-to-month decline. America Online appears to have had the highest incidence of e-mail being delayed overnight. This also affects other ISPs because the saturation or breakdown of one ISP's SMTP servers causes a chain reaction. The mail delivery error messages go back to the sending ISPs and build up large queues. It is the increasing number of messages which are delayed many hours which affect the overall overage. Inverse expects the problem to continue, therefore, one should not assume that an unanswered e-mail message has been delivered.
https://librarytechnology.org/document/5666/