Thanks for the Post.
With Reckon Accounts Hosted, because you are using email program on our server you cannot check the sent email so you cannot confirm which email address tax invoice was sent under.
You can ask the customer to check the spam or Junk email as well and see if it’s not under that.
Otherwise in future, when you send email to customer, always BCC to yourself. So that way you can have the record of invoice being sent.
Another way, which could be a bit lengthy, is to print that Invoice which “saves as PDF”. Once opened, you can use the email button on the viewer to email to the customer. That will use your local PC’s email client and you can always track it just like you do with other local email tasks.
Hope this will help,