The delivery rate for permission-based e-mail marketing campaigns varies by up to 10 percent, depending on the time of day and the day of the week that messages are sent. According to research from Return Path, delivery date and delivery time are just as critical to the success of e-mail marketing campaigns as ISPs and spam filter triggers.

Return Path measured the deliverability of e-mail messages within specific campaigns to determine how different delivery days and times affected the deliverability rate of particular messages within a campaign. The New York City and Superior, Colo. e-mail delivery services firm reviewed more than 16,000 e-mail marketing campaigns between January 1, 2004 and April 31, 2004 to generate a deliverability index. The index includes actual delivery rates for 3.4 million e-mail messages sent during the survey period.

The survey found that campaigns sent between 6 a.m. and 10 a.m. EST had deliverability rates two percent higher than any other time. Campaigns sent between 10 a.m. and 2 p.m. EST were far less successful falling, 3.1 percent below the campaign average. The 10 p.m. to 2 a.m. EST period was the second best period for deliverability, pushing the rate up almost one percent.

The report makes a correlation between successful delivery periods and times with a high incidence of spam. Citing data from SpamCop, Return Path notes that most spam is reported between 9 a.m. and 3 p.m. EST, resulting in lower permission-based e-mail marketing delivery rates.

The survey found that Monday was by far the best day for successful delivery. Campaigns sent on Mondays had a deliverability rate more than one percent higher than the campaign average. Messages delivered on Tuesdays, Wednesdays, and Thursdays showed deliverability increases of less than one-half percent. Messages sent on Fridays, Saturdays, and Sundays performed bellow the campaign average -- Saturday is the worst day to send an e-mail campaign with a deliverability rate more than one percent below average.

The report noted that some spam filters factor in delivery time and day. The example cited was a MAPI Labs analysis that found Outlook 2003 assigned a higher spam weighting to e-mail sent on Saturdays and Sundays.

The survey also combined data on delivery times and days to determine the best and worst day-time combinations to send e-mail campaigns.

Despite the low overall success rate for Saturdays, e-mails sent between 10 p.m. and 2 a.m. EST on Saturdays had a deliverability rate more than 3.5 percent above the average. The leading weekday period was Fridays between 6 a.m. and 10 a.m. EST; deliverability rates jumped two percent during this period.

The worst performing period, more than 10 percent below average, was Sundays between 10 a.m. and 2 p.m. EST. The almost two percent below average deliverability rate on Fridays between 10 a.m. and 2 p.m. EST was the lowest of any weekday period.

"While time and day delivery success should be a key consideration when doing any email campaign, marketers should recognize that it is just one point to test," said George Bilbrey, vice president and general manager of deliverability services at Return Path, in a statement.

Based upon the results of their research, Return Path recommends the following guidelines to improve deliverability rates of permission-based e-mail marketing campaigns: