The solution: Simply create a new procmail rule that copies (that’s what the c stands for) any incoming message to a special folder (I named it “mobile”; note that my postfix uses the maildir format to store messages):

:0 c
.mobile/

Preferably, this rule should be placed after the spam filtering rules (you don’t want to pay for spam on your mobile, do you? ;) and before the normal procmail mail filter rules (which move every incoming mail to the appropriate subfolder). You can also use a dedicated rules file for every group of similar rules. For example, I include the files spam.rc, mobile.rc, lists.rc (with their according rules) in the main .procmailrc config file.

Now every mail that wasn’t filtered by the spam filter gets copied to the “mobile” subfolder of your inbox prior to getting moved to the appropriate subfolder (by the rules in lists.rc).

On my e61i, I hence only need to subscribe to the “mobile” subfolder in order to catch all incoming messages. As you can’t unsubscribe from the inbox subfolder itself, you should set the maximum number of caught new inbox mails to 1 (which is the minimum) in order to avoid fetching things twice. You can further configure that only headers are fetched to minimize the amount of data[1] that is automatically downloaded with every mail check (also: switch off polling if you don’t need it).

Note that another advantage of this solution is that you won’t tamper with any of the original messages (that are either in your inbox or in any of its subfolders). Some might call this a disadvantage though.

That’s it, have fun! :)

[1] I pay 0.10 CHF per 20 KB, i.e. 5 CHF (5 USD) per MB (which is incredibly expensive as I currently don’t have any flat rate data plan -> please don’t send me a mail bomb just for fun, else I’ll have to write a more sophisticated procmail rule that only copies messages < 100 KB to the mobile folder)