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If your friend swears he invited you to that party, but you never got the e-mail, he might just be telling the truth.

For more than three weeks, e-mail sent by users of Google’s popular Gmail service to MIT has been delayed or left undelivered, confounding the common belief that e-mail is a fast, reliable way of sending a message. The problem was simple: Gmail was getting too popular.

The Barracuda Networks machines that filter spam out of MIT’s incoming mail are configured to accept only about a hundred connections an hour from any one server. But Google’s servers were sending mail much faster than that, and MIT told Gmail that “too many connections” were received.

On Tuesday afternoon, MIT Network Manager Jeffrey I. Schiller ’79 implemented a fix which was likely to solve the problem, by telling MIT’s spam firewalls to ignore connection limits when processing mail from Gmail servers.

The errors may reflect the growing popularity of Gmail among MIT community members, who may forward their mail there while continuing to use their username@mit.edu. Google is the largest recipient of MIT mail beyond internal mail servers, according to Schiller: one MIT mail delivery server sent about 35,000 messages a day to an MIT mail server and about 34,000 messages a day to Gmail servers.

Schiller said MIT Information Services and Technology had received about a dozen complaints about the troubles with the Barracuda machines and Gmail servers. He could not say how many messages might have been delayed or misdelivered.

The Barracudas face a daunting problem because they handle several million messages a day. Looking at one of the six machines on Tuesday, Schiller said that 52,800 messages had been delivered, 27,000 had been delivered but marked as possible spam, and 852,000 attempts to send a message had been blocked.

The Barracudas have not always run smoothly. Sometimes they “eat” messages, which may sometimes be delayed for hours or days. At least once a month, spammers target one of the six Barracuda machines and send it tens of thousands of junk messages, paralyzing the machine’s queue; legitimate mail which was passing through that machine may be delayed for as long as it takes to sort out the mess.

Schiller said MIT is testing out an alternative to the Barracuda machines. He declined to comment on specific vendors or prices. A sticking point in prior negotiations has been fees: many anti-spam vendors charge a substantial fee per e-mail user. In exchange for a flat fee, Barracuda mails MIT a black-and-blue box which filters e-mail for them.

Other vendors might provide just the software and let MIT use its own hardware, or they might supply superior “black boxes” — occasional hard drive and motherboard failures have hurt Barracuda reliability. Other antispam software might be easier to monitor: much Barracuda log information can only be viewed on a Web server on the device and cannot be extracted for later inspection.

But until MIT gets itself a better deal, the Barracudas are here to stay.