A US security vendor claims to have discovered the reason for Yahoo’s alleged poor email delivery – as many as half its email hosts can be “closed” at any one time.

According to trimMail’s Email Battles site, which recently monitored 16 of the company’s advertised email hosts 240 times over a half hour period, only 133 of its probes were answered. Many of the servers were closed and unavailable.

Overall availability ranged from 25 percent to 75 percent over the admittedly short test period. The average availability was 55 percent, with the worst of the servers available only 7 percent of the time.

The measurements were taken using the company’s freely-available web tool Mail Server Profiler.

Yahoo struggles with a reputation for mediocre service in some quarters, but it is difficult to know what to make of these figures. The company is likely to counter that their accusers only took the readings over a short and untypical period, or that the hosts being closed was not in itself indicative of service levels.

The site suggests paying customers take their business elsewhere rather than waste their own server resources trying to cope with mail resends.

In a single-query repeat of the Email Battles test, Techworld discovered that six of the sixteen mail hosts were marked as closed, a similar result. By comparison, of the 16 possible hosts on Hotmail, four were found to be closed, while all of Gmail’s eight hosts were up.

It is impossible to say whether this comparison is representative or not, or the fact that the hosts are closed is in itself the reason for poor email service – that would require a much longer and more detailed test. It would also be dependent on traffic volumes at a given moment in time, which can vary.

Yahoo responded to the story with the following comment. "Yahoo! Mail treats e-mail delivery as a top priority. We are successfully delivering billions of e-mail messages per day, and in fact, Yahoo! Mail delivers more messages per quarter than the US Postal Service, FedEx and UPS combined in a year. Yahoo! is constantly enhancing technologies and operations in order to deal with the load."