Microsoft will be disabling basic authentication for Exchange Online email accounts later this year:

https://developer.microsoft.com/en-us/office/blogs/end-of-support-for-basic-authentication-access-to-exchange-online-apis-for-office-365-customers/

This means that you will no longer be able to send emails for invoices from Kashflow if you use Office 365 Exchange Online for you email server.

The same will likely follow suit for other email providers like Google, etc.

Kashflow need to switch to using Modern Authentication