The Challenge
The IT department of a large enterprise with more than 50 locations and dozens of mail servers kept running into the same problem: emails were not arriving, and troubleshooting was tedious. Standard tools such as Telnet or OpenSSL s_client were too cumbersome for everyday use, and commercial SMTP test tools did not cover all requirements. In particular, simultaneous testing of multiple servers, detailed TLS certificate analysis and reproducible test scenarios were missing.
The administrators needed a tool that could be used both in first-level support and for in-depth infrastructure analysis.
Our Solution
We developed a desktop application that brings every aspect of SMTP testing together in a clear interface:
Protocol Support
Comprehensive support for all common SMTP variants:
- SMTP (port 25) for communication between mail servers
- SMTPS (port 465) for implicitly encrypted connections
- STARTTLS (port 587) for opportunistic encryption
- Configurable timeouts, EHLO domains and authentication methods (PLAIN, LOGIN, CRAM-MD5)
Detailed Connection Logging
Every step of the SMTP communication is logged and displayed with color coding:
- Client commands (EHLO, MAIL FROM, RCPT TO, DATA) in blue
- Server responses (250 OK, 354 Go Ahead, 550 User unknown) in green or red depending on the status code
- TLS handshake with the negotiated cipher suite and protocol version
- All logs can be exported as a text file, ideal for support tickets and documentation
Certificate Analysis
After a successful TLS handshake, the server certificate is analyzed in detail:
- Common Name and Subject Alternative Names
- Issuer and certificate chain
- Validity period with a warning before expiry
- Fingerprint (SHA-256) for comparison with expected certificates
- Check for known weaknesses (weak key length, outdated signature algorithms)
Multi-Server Configuration
Management of all the company's mail servers in one place:
- Server profiles with saved connection parameters
- Grouping by location, function or priority
- Quick test for individual servers with a single click
- Import and export of configurations for sharing within the team
Batch Test for Server Lists
For regular infrastructure checks:
- Import server lists as CSV and test all servers one after another
- Results overview with a traffic light system (reachable, TLS problem, unreachable)
- Automatic report as HTML or PDF with all test results
- Ideal for monthly compliance checks and certificate monitoring
Attachment Test Function
Testing email delivery with various attachments:
- Configurable test attachments in different sizes and formats
- Checking size limits and MIME type filters
- Simulation of typical use cases (PDF invoice, Excel report, ZIP archive)
The Result
- Average fault diagnosis reduced from 45 minutes to under 10 minutes
- Batch tests check all 50+ mail servers in under 5 minutes
- Certificate monitoring has already caught expiring certificates in time on three occasions
- Standardized reports simplify communication between IT teams and service providers