How Incoming Email is Processed

ImprovMX implements comprehensive verification procedures when receiving emails. The service validates sender identity, email envelope details, and content through multiple stages of the SMTP protocol.

Connection Phase

When clients connect to mx1.improvmx.com or mx2.improvmx.com on port 25, ImprovMX assigns the connection an IP address and initiates a session timer. Basic infrastructure setup occurs at this stage.

HELO/EHLO Command

Several validation checks occur during this phase:

• IP Blocking — Connections from IPs that exceeded rate limits or generated multiple 500 errors face temporary bans with error message 454 4.7.1...
• Hostname Verification — IP addresses must match provided hostnames; mismatches trigger rejection
• Reverse DNS (PTR) — The service verifies reverse name resolution matches the connecting IP
• DNSBL Checks — The system queries Spamhaus, Sorbs, and SpamCop blacklists

MAIL FROM Command

The return-path (bounce address) is captured but detailed validation defers to the DATA stage. This value can be empty (<>) for bounce notifications.

RCPT TO Command

For Bounces

ImprovMX verifies legitimacy using hashes in return-path emails to prevent backscatter.

For Authenticated Senders

Up to 50 recipients allowed per email with checks including:

• User account existence
• Account status verification
• Daily sending quota enforcement

For Forwarded Emails

Maximum three recipients on the same domain, with domain-level validation including existence confirmation and quota checks.

DATA Stage (Core Processing)

This is where primary validation occurs with the complete email envelope and content.

Initial Checks

• Bounce/delivery report detection
• From header presence and RFC validity
• Forward loop detection

Content Validation

• Attachment extension filtering
• Quota and file size verification
• SPF, DMARC, and DKIM authentication
• SpamAssassin spam detection

Forwarding Verification

• Recipient quota confirmation
• MX record validation for destination addresses
• Queue placement for autoscaled forwarding

The service appends custom headers and signs emails with DKIM before queuing for delivery.