There may be many possible causes. Here they are in order or probability:
The outgoing mail server information in the Settings is incorrect, particularly the SMTP server, username, password, or email port. Please double-check this information and try to send again.
A firewall or other security setting on the computer or network is preventing eShuttle from sending email. Please double-check the firewall and other security settings to make sure that the eShuttle program is allowed to send email on the selected port.
The email account being used does not allow POP or POP3 access. Please double-check with your email service provider to make sure that you do not have a web-only email service.
In some cases, your Internet Service Provider (ISP) may not allow you to send emails through another outgoing server. For example, if you have an Internet connection through Cox, but are using a Yahoo mail account, then Cox may require you to use a Cox outgoing mail server. Please check with your ISP to see if this is the case and to get the correct SMTP server information.
Finally, and very rarely, the email service may not require or allow the password when using an email address from them. For example, if you are sending email through a Bellsouth outgoing mail server, and if you are using an email address that ends in @bellsouth.net, then you the server may not accept the username and password. Please try removing the password completely from the Settings and try to send again. If that does not work, try removing the username as well and sending again.