I'm using python-quickbooks to interact with the QuickBooks Sandbox API, and am able to email an invoice via a US company succesfully, but when I do the same for a GB company, no emails are received. The QuickBooks API doesn't return any errors for the GB company, and the 'EmailStatus' attribute on the Invoice object returned from the POST request to send the email is 'EmailSent'.
All of the responses return a 200 Status Code, and I'm not receiving any errors in response when I make the API calls.
Below is an overview of the requests I'm making to the QuickBooks Sandbox API for the GB sandbox company (company/realm ID 4620816365361166740) which is not working, with the URLs and a summary of the request body contents (the flow to create new customers/invoice/items). The working US sandbox company flow is identical, but the company/realm ID is 4620816365362928020.

I've checked to see if there might be any account settings that would affect sending emails, but couldn't find any settings that might affect this.
Are there any account settings that might affect this that I may have missed and if so, how do I fix this?

I note that there is one difference between the two companies, in that the Sandbox company for US has "PAYMENTS" as "Enabled" and the GB company has "PAYMENTS" as "Not enabled". I couldn't find any settings to enable payments within the GB company sandbox. Could this affect the sending of emails and be the cause of this issue and if so, how do I fix this?

Please let me know if there are any known issues relating to this, or how I might go about fixing this issue?

response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/customer , Status: 200 , Req Method: POST
result: [Customer is in response body]
response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/account , Status: 200 , Req Method: POST
result: [Account is in response body]
response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/account , Status: 200 , Req Method: POST
result: [Account is in response body]
response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/item , Status: 200 , Req Method: POST
result: [Item is in response body]
response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/invoice , Status: 200 , Req Method: POST
result: [Invoice is in response body]
response for: https://sandbox-quickbooks.api.intuit.com/v3/company/4620816365361166740/invoice/268/send?sendTo=James.McIntyre.100%40Gmail.com , Status: 200 , Req Method: POST
result: [Invoice is in response body]