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There has been little research conducted into the current state of SMTP (Simple Mail Transfer Protocol) servers on the Internet, perhaps because this area of research has not been particularly fashionable in comparison to the HTTP metrics which are commonly collected. SMTP is the protocol used to deliver email on the Internet. This is an important area of research however given the level of traffic served by these systems has been growing for years. Barracuda Networks cite Radicati research which indicates that in 2009 228 billion emails will be sent per day, with the vast majority being spam (see Barracuda's site for more details). Afergan and Beverly in "The state of the email address" evaluate the state of email servers in an attempt to determine how SMTP servers are coping with the growth in traffic. Their approach involved sending out probe emails to a variety of domains. The email was crafted to have a strong assurance of bouncing because of not being addressed to a valid address. The authors then monitored the bounce traffic. They concluded that corporate SMTP servers are under surprising levels of strain and do not bounce undeliverable emails in a predictable manner.
This site hosts data derived from my survey of SMTP servers commented to the Internet. The survey takes the form of a simple popularity contest, with around 46 million IP address currently surveyed. The results are based on parsing the "220 status" line that SMTP servers send a client upon connection to TCP port 25. Current results are available, as well as an interactive lookup interface if you're curious about a specific domain name. If you're interested in the methodology for the survey, you should checkout my LISA 2007 poster about this survey which is the most recent public documentation. Its a little out of date at the moment, but the latest version isn't yet ready for publication.
There have been surveys of SMTP servers conducted by others. The table below provides a summary:
| Date |
Surveyor |
Sample size |
Sample method |
Responses |
URL |
| 27 November 1996 |
DJB |
500,000 |
Selective random |
25,121 |
Site |
| 14 August 1997 |
DJB |
200,000 |
Selective random |
8,056 |
Site |
| 11 May 1998 |
DJB |
20,310 |
MX walk |
17,592 |
Site |
| 2 April 2000 |
DJB |
12,595 |
Selective random |
10,087 |
Site |
| 5 October 2000 |
DJB |
25,777 |
Random |
859 |
Site |
| 27 September 2001 |
DJB |
39,206 |
Random |
937 |
Site |
| 1 December 2002 |
Credentia |
4,096 |
Random |
1,837 |
Site |
| 1 January 2003 |
Credentia |
30,000 |
Random |
17,540 |
Site |
| 1 April 2003 |
Credentia |
37,563 |
Random |
20,410 |
Site |
| 1 May 2007 |
MailChannels |
400,000 |
Listing of domain names owned by companies |
254,400 |
O'Reilly article |
| Ongoing |
E-Soft |
Unknown |
Unknown |
Unknown |
Site (data available for a fee) |
| Ongoing |
Michael Still / Eric McCreath |
46,891,390 |
Exhaustive |
1,923,664 |
Site |
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