In some circumstances disabling sub-addressing can improve deliverability.
When a message is sent via Gaggle Mail the From: address is changed to be a @gaggle.email address. The group name and the original sender's address are added to this domain in the form email@example.com.
Adding the address of the original sender on the local part after a '+' sign uses an email feature known as sub-addressing. This is very useful since it gives every sender to the group their own unique address.
However, sometimes in order to avoid spam filters and successfully deliver messages, it is necessary to add the sender's address as a contact in the recipient's email client.
For this to work sub-addressing has to be switched off so that messages from the group always come from the same group address (in our example firstname.lastname@example.org) rather than different addresses from different original senders.
Sub-addressing can be switched off globally for a group from your Settings page.