Invalid Email

I keep getting the following error when attempting to send an email via function. It seems to only occur when a certain email address is used in the from fields (rec.Return_Email and rec.CC_Email) however it is definitively a valid email address so I'm not sure why this happening.

Error details:

Error in executing sendemail2 workflow.

Error.

Error in executing Send Mail task. InValid email Address Found.

Here is the function:

void sendemail2(Add_a_Container records)
{
    msg_content = "<table border=\"0\" style=\"\" width=\"90%\" cellpadding=\"5\">\n    <tbody>\n        <tr>\n            <th align=\"left\" bgcolor=\"#bdbdbd\"><span style=\"color: rgb(255, 255, 255); font-family:Arial; font-size:11pt; \">Origin</span></th>\n     <th align=\"left\" bgcolor=\"#bdbdbd\"><span style=\"color: rgb(255, 255, 255); font-family:Arial; font-size:11pt; \">Warehouse</span></th>\n       <th align=\"left\" bgcolor=\"#bdbdbd\"><span style=\"color: rgb(255, 255, 255); font-family:Arial; font-size:11pt; \">Container Number</span></th>\n            <th align=\"left\" bgcolor=\"#bdbdbd\"><span style=\"color: rgb(255, 255, 255); font-family:Arial; font-size:11pt; \">Vessel</span></th>\n                   </tr>\n        ";
    val = "";
    for each rec in records
    {
        val = ((((((((val + "<tr>\n            <td bgcolor=\"#ededed\"><span style=\"font-family:Arial; font-size:11pt; \">") + rec.Origin) + "</span></td>\n     <td bgcolor=\"#ededed\"><span style=\"font-family:Arial; font-size:11pt; \">") + rec.Destination) + "</span></td>\n       <td bgcolor=\"#ededed\"><span style=\"font-family:Arial; font-size:11pt; \">") + rec.Container_Number) + "</span></td>\n        <td bgcolor=\"#ededed\"><span style=\"font-family:Arial; font-size:11pt; \">") + rec.Vessel) + "</span></td>\n            </tr>";
        rec.Return_Alert = "Dispatch Email Sent";
    }
    msg_content = msg_content + val + "\n    </tbody>\n</table></font>";
    sendmail
    (
        To       : rec.Return_Email
        From     : zoho.loginuserid
        CC       : rec.CC_Email, zoho.loginuserid, zoho.adminuserid,
        Subject : "Containers Ready for Pick Up - LT Custom"
        Message : ("The following containers are empty and ready for pick up at the listed warehouse(s).<br />\n<br />\n") + msg_content
    )
}

Anyone have any ideas?