The worst time to send an email is between 5pm and 6pm on Thursdays, research has found.
Web performance monitoring firm Epitiro said that an estimated seven per cent of emails are either severely delayed or lost altogether during this 'rush hour'.
This is due to the number of people using the web at that particular time, Epitiro said.
And it's not just the day of the week or time of day that can affect email delivery times - the time of year is also a factor too.
Epitiro found that in the autumn months, email delivery times got slower, with eight per cent taking longer than three minutes to arrive.
"Email delivery speed is a very important factor in internet communications, not least for the credibility of an online business," said Gavin John of Epitiro.
"When customers shopping online are told to await email confirmation of a purchase, they expect this to arrive in no more than a couple of minutes. If an email takes longer than three minutes to arrive, many customers will worry that something has gone wrong," Johns continued.