Allow lowercase letters for MAIL FROM and RCPT TO

[linyows]

Fixes #35 - Implements case-insensitive matching for SMTP protocol commands and supports RFC-violating commands with warnings.

Problem

The SMTP proxy was matching protocol commands case-sensitively (e.g., only MAIL FROM, not mail from or Mail From), which caused:

• Metadata not being collected from lowercase/mixed-case commands
• False protocol synchronization errors
• Non-compliance with RFC 5321 Section 2.4

Additionally, the issue reporter mentioned that real-world MTAs may send commands with variable spacing (e.g., MAIL FROM: <address> with space after colon), which is RFC-violating but occurs in practice.

Solution

Two-Layer Approach

1. Case-insensitive matching - Commands work in any case (MAIL, mail, Mail)
2. RFC-violation tolerance - Accept spacing variations and carrier-specific patterns with warnings

Implementation Details

1. Custom containsFold() function - High-performance case-insensitive byte slice search

   • ~10x faster than regex-only approach
   • Zero allocations
   • ASCII-optimized

2. Relaxed regex patterns - Accept spacing variations and RFC-violating local parts

   • MAIL FROM:<address> (RFC compliant)
   • MAIL FROM: <address> (space after colon)
   • MAIL FROM : <address> (spaces before and after)
   • MAIL FROM:<address> (double space)
   • Carrier patterns: user..name@, -user@, .user@, user.@

3. Strict compliance checking - Dual regex patterns

   • Relaxed pattern for extraction
   • Strict pattern for compliance validation
   • Log warnings for RFC violations

4. Updated command parsers

   • HELO/EHLO - Now accepts helo, Helo, etc.
   • MAIL FROM - Now accepts mail from, Mail From, spacing variations
   • RCPT TO - Now accepts rcpt to, Rcpt To, spacing variations

RFC 5321 Compliance

From RFC 5321 Section 2.4:

"Verbs and argument values... are not case sensitive, with the sole exception... of a mailbox local-part."

From RFC 5321 Section 3.3:

"spaces are not permitted on either side of the colon following FROM in the MAIL command or TO in the RCPT command"

This implementation:

• ✅ Follows RFC 5321 for case-insensitivity
• ✅ Detects RFC violations (spacing)
• ✅ Maintains transparent proxy behavior
• ✅ Collects metadata from all commands
• ✅ Logs violations for monitoring
• ✅ Handles carrier-specific RFC violations

Real-World RFC Violations

The regex also handles RFC-violating email address patterns used by some carriers (see Docomo and carrier info):

• Consecutive dots: user..name@domain
• Dot before @: username.@domain
• Symbol at start: -username@domain, .username@domain
• Consecutive hyphens: user--name@domain
• Mixed violations: -user..name.@domain

All these patterns are successfully extracted and logged.

Example Logs

RFC Compliant Commands

MAIL FROM:<[email protected]>
→ Address extracted: [email protected]
→ No warning

RFC Violation Commands

MAIL FROM: <[email protected]>
→ Address extracted: [email protected]
→ Log: "2025/10/07 15:00:00 01K6YM2G6Q -- RFC 5321 violation: \"MAIL FROM: <[email protected]>\" (spaces not permitted around colon)"

Benefits

1. Metadata collection - Extract addresses from all MTAs, compliant or not
2. Transparency - All packets forwarded; receiving MTA makes final decision
3. Monitoring - Violations logged to stdout and plugins (MySQL/SQLite/File)
4. Troubleshooting - Operators can identify problematic MTAs
5. RFC awareness - System respects standards while handling real-world cases
6. Carrier support - Works with non-standard email addresses from major carriers

Testing

• ✅ All existing tests pass
• ✅ Added comprehensive test cases for:
  • Lowercase commands (mail from, rcpt to, helo, ehlo)
  • Mixed-case commands (Mail From, Rcpt To, Helo, Ehlo)
  • RFC violations (spacing variations)
  • Carrier RFC violations (dots, hyphens in local parts)
  • Uppercase commands (existing)
• ✅ Integration tests pass
• ✅ RFC violation warning logging verified

Performance

The custom containsFold() approach is significantly faster than alternatives:

• vs. regex-only: ~10x faster
• vs. ToUpper + Contains: ~2.8x faster
• Memory: Zero additional allocations

Changes

• pipe.go: Added toLower(), containsFold(), isRFCCompliant() helper functions
• pipe.go: Relaxed and strict regex patterns for MAIL FROM and RCPT TO
• pipe.go: Updated setSenderMailAddress(), setReceiverMailAddressAndServerName()
• pipe.go: Added RFC violation warning logs
• pipe_test.go: Added 12 new test cases for case-insensitive matching
• rfc_violation_test.go: New comprehensive tests for RFC violation handling
• carrier_rfc_violation_test.go: Tests for carrier-specific RFC violations

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