The only reason Salesforce is trying to serialize the `List<Messaging.SingleEmailMessage>` property is because you are adding it to the view state. You don't need to, however, as you do not reference it in your markup. Simply **remove** the `messageList` property and scope it locally. If you really want to keep it as a property, mark it as `transient`. 

##Repro

The MVR is simple here:

**Page**

    <apex:page controller="DemoExtension">
        <apex:form />
    </apex:page>

**Controller**

    public with sharing class DemoExtension
    {
        public List<Messaging.SingleEmailMessage> messages { get; private set; }
        public DemoExtension()
        {
            messages = new List<Messaging.SingleEmailMessage>
            {
                    new Messaging.SingleEmailMessage()
            };
        }
    }

##Fix

Marking `messages` as `transient` resolves the error, as does removing it as a property and declaring it only in the constructor.

**Transient**

    public with sharing class DemoExtension
    {
        public transient List<Messaging.SingleEmailMessage> messages { get; private set; }
        public DemoExtension()
        {
            messages = new List<Messaging.SingleEmailMessage>
            {
                    new Messaging.SingleEmailMessage()
            };
        }
    }

**Locally scoped**

    public with sharing class DemoExtension
    {
        public DemoExtension()
        {
            List<Messaging.SingleEmailMessage> messages = new List<Messaging.SingleEmailMessage>
            {
                    new Messaging.SingleEmailMessage()
            };
        }
    }