In such cases, the receiving server can notify you of difficulties using SMTP error codes, and realizing what they necessarily mean can help you diagnose and repair email shipping and delivery roadblocks. As an example, Here's two teams of SMTP errors that crop up normally: By using interactive emails, https://www.upwork.com/services/product/development-it-your-company-s-private-network-vpn-setup-for-remote-work-1447284676343549952?ref=project_share&tier=0