View Single Post
I have set up Mail such that on deletion the mails are moved into the Trash folder and then automatically deleted after one week (Preferences -> Accounts -> Mailbox Behaviors).

Thus, I can either file or delete a mail after recording it into OmniFocus. The only pitfall is when I don't complete the task corresponding to a deleted email within one week - but then again, the whole email text is in the task annotation, so I'm safe here as well.