It's important that your transactions are correctly entered for your books to be well balanced. QuickBooks Online is a great tool able to help you send invoices to your customers so they know the amount owed. I can point you in the right direction for assistance with setting up safe senders.
in order to set up an email as a safe sender, the receiving party of an email would need to go to the settings of their email account to add them as a safe sender, and this can differ depending on your email provider. I recommend reaching out to your email provider to know how you can set up your own suppliers as safe senders, and if it's your customers that need t set you up as a safe sender, they'd need to check with their email provider.
If you have any other questions about QuickBooks, feel free to reach out here.