C# Receive Email: Build an Inbound Handler in ASP.NET Core

Receiving email in C# involves two distinct scenarios: reading messages from an existing mailbox through IMAP, POP3, Microsoft Graph, or Gmail APIs, or making an application itself the email destination through an inbound email service and webhook. For application-directed email, the described Mailtrap workflow provisions a hosted or custom-domain inbox, verifies webhook HMAC signatures against the unmodified request body, uses webhook message IDs to retrieve full parsed messages, and processes fields such as text_body, html_body, recipients, thread IDs, and nullable attachments. Attachment download URLs are temporary and should be fetched during processing, while oversized messages can be rejected before storage or webhook delivery. The approach can support ticketing systems, automated workflows, and AI agents that use conversation threads to read, reply, or forward messages, with outbound sandbox testing available for end-to-end validation. For mailbox access, MailKit is recommended for IMAP or POP3, Microsoft Graph with OAuth2 is preferred for Microsoft 365, and Gmail generally requires OAuth2 or limited app-password use, while System.Net.Mail’s SmtpClient only sends email and cannot receive it.

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