At the same time, its results can be easily measured, which helps marketers fine tune future communications based on their target audiences and product offerings.

Not surprisingly, as far back as 2011, 88% of businesses that pitch products to consumers used email as part of their marketing, according to Forrester Research. Yet, much like paper junk mail, email marketing can be challenging as some recipients will inevitably send the communications to an untimely cyber death with the single click of the delete key.

One way to prevent that from happening is to send email during times when clients and prospects are most likely to read the communications. Timing is important, in part, because emails are most likely to be opened within an hour of being sent—as time passes, email recipients become increasingly less likely to open the communications.

The tricky part, of course, is figuring out when clients and prospects are most likely to open emails. It’s easier to identify time periods when recipients are least likely to open emails.

For example, from 4 p.m. until 8 p.m. is generally a poor time to send emails as individuals during that timeframe are preoccupied—first with leaving their workplace at a reasonable hour and then with completing the commute home as quickly as possible. Some commuters who take public transit may be able to check emails during their trips home, but in urban settings many may not have wireless connectivity while traveling on subways.

Workday hours may be the best time to send emails. According to email marketing firm MailChimp, more people open emails during the day than at night. More specifically, the largest volume of emails occurs on Tuesdays and Thursdays, so email senders may face less competition for readers’ attention on other days of the week.

While weekdays may be a sweet spot, readers are also fairly likely to open emails in the late evening hours. In a VerticalResponse study, emails sent between 8 p.m. and 11:49 p.m. had a 21.7% open rate. The time period is also appealing because readers are likely to “click through,” or open links included in the emails, during the evening hours.

VerticalReponse has also determined that weekends can be productive for email campaigns. Broadly speaking, a lower volume of emails is distributed on weekends, even though readers are more likely to open the communications during the non-work days. Emails sent on weekends, like during weekday evenings, also have a higher click-through rate.

With the low-cost nature of email in mind, it’s easy for marketers to get carried away with sending out large numbers of electronic communications, but in doing so, marketers risk overwhelming and alienating clients. With that in mind, the frequency of sending emails is probably just as important as timing.

Indeed, in an ExactTarget survey, half of individuals who unsubscribed from email communications said they did so because they were getting too many electronic communications.

In the same study, 22% of individuals said they stopped buying products from firms that overwhelmed them with too many emails. To avoid overwhelming subscribers, financial marketers should base the frequency of their emails on the content of their communications. For example, emails that provide value, such as notifications that statements or market commentaries are ready, are less likely to alienate subscribers, unlike emails that are intended to promote services or specific products.

Marketers should also be aware of the CAN-SPAM Act, which regulates email practices. Among other requirements, it stipulates that emails offer recipients an opportunity to opt out of receiving future emails. It also stipulates that firms must disclose their addresses in their marketing emails and it prohibits making inaccurate or misleading statements in headers or subject lines.

For marketers, the bottom line is that there are no hard and fast rules for the optimal frequency and timing of sending emails. Rather, firms should ensure that they have appropriate email tracking services so that they can measure responses to their communications. Armed with that information, firms canconstantly assess and fine tune when they distribute their electronic communications.