Hey! I am using the spanish version of Wysija 2.6.16 and I just found a bug together with my server admin:
In Settings->Advance, when it asks for an email adress for rebound mail ("correo rebotado"), it is actually asking the validation email for SMTP service.
In previous versions I had it configured with a Gmail account, so rebound notifications could reach my e-mail directly, but in this version it is trying to log in to my SMTP service with that same Gmail account.
I hope to be clear and helpful! Cheers for Wysija!
Gonzalo