For several weeks, said , vice president of information security at Colorado-based MX Logic, his company has been receiving complaints from customers about messages from the Obama presidential campaign. MX Logic started blocking many of those messages "relatively recently," said Masiello, who declined to be more specific about the timing.

Other anti-spam and messaging services have taken similar steps, Masiello said. He would not name the other vendors, but said that "based on conversations with our peers, they've had to take the same stance."

The complaints stemmed from messages that concerned rallies and other political events scheduled in other states, said Masiello. "From the standpoint of the users' perspectives, I can see why some messages are considered spam," he said. "The messages are not really relevant to the individual receiving them. You may live in Colorado, but you may be receiving mail about rallies in Virginia or rallies in Ohio."

People who sign up to receive messages from the Obama campaign provide their e-mail address and their zip code. The latter, said Masiello, should be enough for the campaign to better target recipients.

Masiello acknowledged that users must opt in to receive messages from the Obama campaign. "But when you look at it from a users' perspective, they want content that's relevant to them," he said.