If an invoice can't be sent, you'll immediately get a task for this. Below are the possible solutions to this problem.
Invoice did not arrive
Unable to send an email: Attachments too big. The total size should be less than 10485760 bytes. (code 300).
Solution: the email is too large to send. The problem may be that the attachments are too large, but it could also be that an image included in the template is too large. A photo that hasn't been compressed and/or resized can be very large. Resize the image and/or compress it and replace the image in the template.