You need to set the email account to use an IAP (Internet Access Point) automatically. Please refer to below steps:
Go to Messaging Application -> "Options" -> "Settings" -> "E-mail" -> "Mailboxes" -> "Your default account" -> "Connection settings" -> "Outgoing e-mail" -> "Access point in use", select your 3G/GPRS access point.
Go to "Tools" -> "Settings" -> "Connections" -> "Internet" -> "Your access point name" -> "Use access point", set it to "Automatically".

If an email failed to be sent out for some reason and stays in outbox, Sms2Web will try to send it again when a new forward email is generated. The previous email and new email will be sent together.