Kolleno cannot know before fetching the PDFs whether they are available, so it cannot be done while they are being selected.

However, Kolleno can create a general setting for your Company that the email will still send, even if the Invoice(s) cannot be fetched when sending manual emails.

If you would like this feature, please email support@kolleno.com

Please note that for automated emails sent via Workflows, you can just decide the setting on the Template editor.

See article here.