Globally override Web Forms for Marketers Email Sending settings.

ApplySendingSettings.cs

using System;
using System.Net;
using Sitecore.Form.Core.Pipelines.ProcessMessage;

namespace MyProject.Forms
{
    /// <summary>
    /// Overrides Web forms for marketers mail settings through global configuration options.
    /// </summary>
    public class ApplySendingSettings
    {
        public string Host
        {
            get { return Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.Host", string.Empty); }
        }

        public NetworkCredential Credentials
        {
            get
            {
                return new NetworkCredential(Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.User", string.Empty),
                                             Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.Password",
                                                                                        string.Empty));
            }
        }

        public bool EnableSsl
        {
            get
            {
                var setting = Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.EnableSSL", string.Empty);
                if (string.IsNullOrEmpty(setting))
                {
                    return false;
                }

                return setting.Equals("true", StringComparison.CurrentCultureIgnoreCase);
            }
        }

        public bool SendAsHtml
        {
            get
            {
                var setting = Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.SendAsHtml", string.Empty);
                if (string.IsNullOrEmpty(setting))
                {
                    return false;
                }

                return setting.Equals("true", StringComparison.CurrentCultureIgnoreCase);
            }
        }

        public int Port
        {
            get
            {
                var setting = Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.Port", "25");

                int port;
                int.TryParse(setting, out port);
                return port;
            }
        }

        public string From
        {
            get { return Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.From", string.Empty); }
        }

        public string Bcc
        {
            get { return Sitecore.Configuration.Settings.GetSetting("WFM.Custom.Email.Bcc", string.Empty); }
        }

        public void Process(ProcessMessageArgs args)
        {
            args.Host = Host;
            args.Credentials = Credentials;
            args.EnableSsl = EnableSsl;
            args.Port = Port;
            args.IsBodyHtml = SendAsHtml;
            args.From = string.IsNullOrEmpty(From) ? args.From : From;

            // Add a global BCC
            if (!string.IsNullOrEmpty(Bcc))
            {
                if (args.BCC.Length != 0)
                {
                    args.BCC.Append(",");
                }

                args.BCC.Append(Bcc);
            }
        }
    }
}

Sitecore.Forms.config

<pipelines>
  <processMessage>
    <!-- Custom E-Mail Processor for Web Forms -->
    <processor type="MyProject.Forms.EMail.ApplySendingSettings, MyProject" method="Process" patch:before="processor[@method='SendEmail']"/>
  </processMessage>
</pipelines>
    
<settings>

<!-- Mail Server Host Name / Port -->
<setting name="WFM.Custom.Email.Host" value=""/>
<setting name="WFM.Custom.Email.Port" value="25"/>

<!-- Mail Server login credentials -->
<setting name="WFM.Custom.Email.User" value=""/>
<setting name="WFM.Custom.Email.Password" value=""/>

<setting name="WFM.Custom.Email.EnableSSL" value="false"/>
<setting name="WFM.Custom.Email.SendAsHtml" value="true"/>

<!-- From address to use unless specified otherwise in save action -->
<setting name="WFM.Custom.Email.From" value=""/>

<!-- Global BCC address added to every message -->
<setting name="WFM.Custom.Email.Bcc" value=""/>

</settings>