not able to send email with email template

Try this:

So the problem was: !Mage::getStoreConfigFlag('system/smtp/disable') which you can fix up in Admin > System > Configuration > Advanced > System > Mail Sending Settings and change Disable Email Communications to No so the emails are NOT disabled.

Explanation

Look at the /app/code/core/Mage/Core/Model/Email/Template.php.The code throwing this error is:

if (!$this->isValidForSend()) {
    Mage::logException(new Exception('This letter cannot be sent.')); // translation is intentionally omitted
    return false;
}

Look at isValidForSend():

public function isValidForSend()
{
    return !Mage::getStoreConfigFlag('system/smtp/disable')
        && $this->getSenderName()
        && $this->getSenderEmail()
        && $this->getTemplateSubject();
}

More info.

For me the Problem was, that I defined a custom email template via config.xml of my own module and forgot to set the comment block with subject and so on.

<!--@subject Welcome, {{var customer.name}}! @-->
<!--@vars
{"store url=\"\"":"Store Url",
"var logo_url":"Email Logo Image Url",
"htmlescape var=$customer.name":"Customer Name",
"store url=\"customer/account/\"":"Customer Account Url",
"var customer.email":"Customer Email",
"htmlescape var=$customer.password":"Customer Password"}
@-->

<!--@styles
@-->