• E-Mails to T-Online Addresses are Not Delivered
• IMAP: POP before SMTP ("relay access denied")
• IMAP: Fallback-recipient for unknown users
• The Postfix Parameter "mynetworks"
• IMAP : Serving multiple Domains with the IMAP Server
• Setting Up POP Before SMTP
• Sending E-Mail to External Domains: Relay Access Denied (Openexchange Server 4)