Important note This issue is resolved in Advanced Invoice Layout Version 2.2.0. Please go to your customer account to check if you can upgrade. Short explanation To realize the email attachment functionality in our Advanced Invoice Layout Extension we had to extend the Magento core email logic to add methods for attaching documents. Unfortunately there is no way to do this without conflicts with other extensions that do the same, e.g. Ebizmarts_Mandrill or Aschroder_SMTPPro. Solution To solve this problem the following manual steps are required: Make the Advanced Invoice Layout extension depend on the other extension by creating a new file app/etc/modules/ZVianetz_AdvancedInvoiceLayout_Compatibility.xml with the following content (replace OtherVendor_OtherExtension with the extension name) Copy the file app/code/community/Vianetz/AdvancedInvoiceLayout/Model/Email/Template to app/code/local/Vianetz/AdvancedInvoiceLayout/Model/Email/Template and replace the beginning with Vianetz_AdvancedInvoiceLayout_Model_Email_Template extends Ebizmarts_Mandrill_Model_Email_Template Flush the Magento Cache Please find also further general discussions in this support thread.