Will you install MOM Notification Workflow Solution Accelerator? When you download it from Microsoft webpage you have the option to also download a hotfix. Do not install that hotfix until you have experience the issue. I have done some tests and it seems that if you don’t have the issue, you will get it if you install the hotfix.
Install NTWF according to the installation documentation that is included in the package. Then setup SMTP according to the MS KB. If you already have installed the hotfix and suspect that it is generating problems, uninstall it according to the last page in the document that is included in the package.