Rainy days and Mondays may have always gotten Karen Carpenter down, but according to a new study by Return Path, Monday a real "kick-in-the-pants" for permission-based email marketers.

The report shows that mail sent on Monday garners a delivery rate 1% above a company's average, and one that's twice as good as rates for Tuesday, Wednesday and Thursday, which have historically been considered the best days to mail.

According to the study overall deliverability rates for permission-based email campaigns can vary as much as 10 percent, based purely on the date and time mailings occur. Weekends are generally consider anathema because some spam filters more closely associate weekend mailings with spam.

Why the sudden shift to Monday? The report doesn't really say. What it does do is suggest some guidelines for marketers:

1. Mail on weekdays, not weekends.
2. If mailings are sent on weekends, the best times are between 6-10 a.m. ET or between 10 p.m. - 2 a.m. ET
3. Email campaigns sent between 6-10 a.m. ET achieve higher rates than any other time period across all days
4. Monday mailings receive higher overall delivery rates, followed by Tuesday, Thursday, and Wednesday.