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SLmail -- Entry to mid-level mail server for WinNT (with an older Win9x version available)

By ServerWatch Staff (Send Email)
Posted Jun 5, 2002


With some useful content filters and anti-spam controls but lacking support for IMAP4, SLmail is a decent, though ultimately limited mail server. In addition, a single-minded devotion to Windows NT (there's no support for other operating systems other than Windows 95/98, and the Windows 9x version is not up to date) makes the SMTP/POP3 SLmail mail server a less-than-ideal tool on the enterprise level, where cross-platform tools are in demand.

On the plus side, SLmail is very customizable: Seattle Lab Software provides a Software Developer's Kit for creating and editing filters. The SDK includes templates and sample source code. While you probably don't want to spend a lot of time programming your own filters, your best bet is to take an example and alter it for your needs. In addition, SLmail's open APIs allow mail to interface with other outside tools, including legacy mail systems and pagers. With some useful content filters and anti-spam controls but lacking support for IMAP4, SLmail is a decent, though ultimately limited mail server.

The administration tools in SLmail are excellent. The entire mail server can be administered from a Web browser -- meaning that it can be administered from anywhere on the Internet -- and it can also be administered from a Windows NT console.

Once installed, SLmail can handle an unlimited number of user accounts and list servers; your only limitation is disk space and (possibly) bandwidth. SLmail supports five types of accounts -- user, alias, forward, auto-responder, and list -- which cover the most commonly used functions. Most e-mail servers are priced on a tiered system, where lower-traffic sites would pay more for the Web server. With a list price of $499 -- which is a lot for a mail server, when you consider that the high-performance and cross-platform CommuniGate Pro server is available in a free ad-supported edition and the ubiquitous sendmail server is completely free -- Seattle Labs would do well to considered a tiered system where small businesses (especially the kind that are actually using Windows NT, as opposed to the enterprises that Microsoft keeps targeting with mixed success) would find that SLmail is an affordable solution.

There are several situations where SLmail may be the superior tool in the enterprise, however. Since SLmail is built around the Microsoft Component Object Model (COM), it can be easily set up as one tool in a larger scheme where COM is the connecting mechanism. Seattle Lab Software claims that since SLmail was developed for Windows 2000 -- not specifically for Windows NT 4.0 -- it can deliver better performance via I/O completion ports and a new thread-pooling algorithm.

In our tests, we set up SLmail behind a firewall and had no problems configuring the mail server quickly. Seattle Labs Software has made an effort to make its software work in general with gateways and firewalls, and the effort shows. In addition, SLmail supports multiple domains.

One area where mail servers must succeed in order to be taken seriously is in spam and junk-mail management. SLmail comes with several filters designed to stop spam. You can set a filter to deny mail from specific individuals, domains, or IP ranges. Similarly, you can set up filters to allow mail only from specific individuals, domains, or IP ranges. You can also tie into the Mail Abuse Protection System's (MAPS) Real-Time Blacklist. Harsh? Perhaps, but users hate spam mail. In addition, relay filters can be used to prevent spammers from relaying junk mail from your system.

As for technical support, Seattle Labs offers 60 days of free voice support and excellent online help. Premium support contracts can be purchased for an additional $149 per year. Premium support customers receive priority attention and an unlimited number of e-mail and telephone inquiries.

Although a respectable product, when compared to competing Windows NT-based servers like NTMail, PostOffice, and IMail Server, SLmail falls short in several key areas, with the lack of support for IMAP4, LDAP, and S/MIME services being the biggest disappointment. While SLmail may be a good tool under very specific circumstances (such as in a purely Windows NT shop where IMAP4 isn't a consideration), it's not a good tool for the enterprise or in situations where IMAP4 support is needed.

Pros: 7 Junk mail reduction capabilities; 7 NT user database import wizard; 7 UUencode, POP3, SMTP mail support; 7 Web-based remote administration capabilities (only available in the 3.x release for Windows NT)

Cons: 7 No Mac/Unix versions; 7 Lacks support for LDAP, IMAP4, S/MIME

New in v2.7: Windows 95/98-only release. Improved CC/Return Receipt handling and Router handling of invalid data, five minor bug fixes (e-mail client compatibility issues, CHKMAIL, MX lookup routines, ISO/MIME header handling, and Routing Exception Indexing problems); Notes
Upgrade Meter: 2

New in v3.x: Improved Anti-Spam protection, content filters, performance enhancements, NT User Database import wizard, full support for Windows NT 5.0 (v3.0); Web-based remote administration, Java telnet client, user password changer, support for virtual domains (v3.1); Release Notes
Upgrade Meter: 5

New in v4.0: Real-Time Blackhole List (RBL) Filter now installed by default; several issues within the "Verify SLmail Registry" utility fixed; security vulnerability within SLmail as reported by eEye on the NT-Bugtraq list fixed; various Web-based administration compatibility issues with Internet Explorer 5.0 and Netscape fixed; Relay Filter now properly handles RFC 1123 syntax properly, is no longer case-sensitive, and domains with routing exceptions are considered "local"; issue regarding Reply-To addresses and mailing lists fixed; Release Notes
Upgrade Meter: 5

New in v4.1: Fixed several issues that ocurred when formatting the maillog.txt file; fixed several locking issues that could lead to program instability and "CRITICAL-ERROR"s; fixed several issues that could lead to SLMAIL.EXE using 100% CPU and allowing .IN files to build up; fixed an issue where SLmail could get into a loop while talking to another SMTP server; restored the Control Panel applet, leaving Relay/Mail Filtering available only via the Web interface; fixed a series of issues within the Web-based administration: Error Saving Under Access Tab, Settings Under Routing Exceptions, Improved Detection and Handling of the DNS and Alias Fields, Now Allows "&" Within User IDs; added support for changing the inbound SMTP port; made several improvements to the mail exchange record handling; fixed several issues that could cause mailboxes to stay locked; fixed an issue where digest processing for a mailing list could loop; improved duplicate list-member checking within list-member import functions; fixed an issue where duplicate UIDLs could be shown within the POP3 server; fixed a series of issues within the Verify SLmail Registry utility: Corrupted Data in report, Improved SmartHost Checking, Improved Alias Handling, Improved DNS Checking, Removed Forward Warnings; fixed an issue that could cause log rollovers to occur at improper times; Release Notes
Upgrade Meter: 2

New in v5.0: Mailbox size limits; support for multiple domains; filtering capabilities, including content and attachment filters that can be configured to keep e-mails with specific words and specific files/file types (like known viruses) from entering or leaving the mail system; Release Notes
Upgrade Meter: 4

Versions Reviewed: 3.2
Reviewed by: Reichard/Stroud
Last Updated: 3/15/01
Date of Original Review: 3/1/98


Operating Systems / Latest Versions:

Windows NT: Intel, Alpha - v3.2 Build 3113
Windows 95/98 - v2.7 Build 1173



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