In its latest study, the DMIS found that 6.2 items out of a possible 12 in the average weekly postbag comes from direct mail campaigns.
More than half of consumers, or 56%, have purchased products advertised through a direct mailing although 40% of direct mail is left unopened.
Credit card campaigns accounted for 25% of the total volume, followed by insurance as the second most popular at 14%. Banking promotions accounted for 10% of the total volume of direct campaigns.
The DMIS cited value for money and convenience as the reason why most consumers are most likely to have bought through direct mail.
Jo Howard-Brown, DMIS managing director, said: "The current high level of receipt is affecting the consumer's treatment and attitude towards direct mail, but without necessarily diluting their responsiveness and purchasing from the medium.
"If direct mail users can maintain the relevance and quality of targeting, the medium can remain effective even as it becomes more crowded."
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