The Sympl documentation for Mail config says:
Note that when using external email addresses which are not hosted on the same server, then the mail is forwarded on directly without the sender being rewritten, which may cause delivery issues if the sender or recipient are using any anti-spam measures.
For this reason, it is generally best to either deliver to a local mailbox which is collected by the user, or to use a Sieve forward rule in a local mailbox to ensure delivery.
Sadly the link to the Wiki page is empty.
I just wondered if anyone has a handy recipe for this? I presume that the idea is to avoid spf (and DKIM?) problems by setting the envelope sender to the mailbox owner.