How to automatically add a task to Astrid?
This is not a really satisfying solution, but it is still better than nothing (really disappointed by Astrid, BTW):
According to the author, the App "Locale SendSilentMail Plug-In" by Stafan Dörr maintains a queue, so Mails are not lost if there is no connection to the mail server. This is sufficient for my cause, though not very elegant.