It was the syntax change that fixed it for us across the board. We had over 1000 customers in our CRM in QB that we needed to update the main email for, our billers like to have multiple emails as the options for who gets the invoice, then depending on the specific invoice they select the appropriate email address from the 'To' drop down menu within the 'Send Forms' box. However, we would get the network connection error which is incredibly unhelpful as someone trying to solve the problem I immediatly think oh there is an issue with the connection to the email server... WRONG, it's due the way we enter multiple emails in our CRM, we had to replace every semicolon and space that was separating email addresses in the 'Main Email' field with a single comma and no space. That fixed it for us but it was a pain to figure out and no notice from QB about the syntax change is not cool.
Now I realize this may not help you in your specific case since your invoice emails seem to be working for every customer except one, this indicates an issue with that single customer's address and not your own email settings/connection.
If I were you I would try these things:
1. Is the email address your sending to even valid? Test this by sending them an email outside of QB from your regular mail client. Try to use the email account that QB uses for sending your invoices.
2. Check spelling and syntax. I would check in your QB CRM that the customer 'Main Email' addresses are separated by a single comma and no spaces.
I hope this helps...
