Gmail has become very strict on emails now so here is something you may have to do if your customers are not receiving your invoices and statements.
The best solution is to send an initial plain mail to your client list asking them to add their address to their white list or mark it as 'always accept email from this address' or whatever mechanism their particular email software allows to mark your emails as safe.
Sample email to send your customers:
Dear (customer name)
We have received emails and calls from a number of people indicating that they are not receiving the emails we have been sending out.
The most likely reason for this is that your email program or ISP is filtering out our emails as spam.
Can you please check your Junk, Spam or Promotions folder for emails from (your company email address)?
Add the following email address(s) (your company email address) to your "white list" or "safe senders" list or however your particular email program allows you to indicate particular email addresses that you want to receive email from:
Watch this video to see how to do it for gmail.