If SliQ Invoicing is failing to send an email, e.g. to email an invoice to a customer, check the error message output. If the error message indicates that either SliQ or the email system was unable to find an attachment, check that any standard attachment you have added to your email template or the invoice in SliQ is actually present on disk. If the attachment is no longer present, either restore the attachment to the location you have specified in your email template or invoice, or update the email template or invoice to remove the attachment. Then try emailing again.
Help on attachments can be found here:
Help on email templates can be found here: