pop users with PDAs getting duplicate mails
We've got a debian server running postfix, it's been going for a good few years now. Recently we've been getting complaints from pop users that are using blackberries and pdas and the like. Now and again they receive a bunch of mails that they have already received, sometimes several weeks or months old.
Usually we tell them to upgrade to an actual imap account and that sorts the problem.
However, there is one bunch that refuse to change to imap because they say the connection is "shaky" (translation, costs money) and they say it's just happened to everyone using an iphone or pda or blackberry.
I don't think they're lying (exaggerating a little perhaps) but I don't see anything unusual in the logs, just the usual deliveries and login/logout. Has anyone ever had a similar problem? Or does anyone have any idea what I should be looking for in diagnosing this?