One of my clients is trying to send an email to another server, but he keeps getting:
550 Sender verify failed
For what I have understood, the other server has the "callout" options active. Thats fine by me, since my clients mailbox is created and exists. But why can't the other server verify the sender?
Do I need to activate any option on my side?
I do not have the "Verify the existence of email senders" or "Use callouts to verify the existence of email senders" options active.
Thank you for your help