Hello Juanmata!
Thanks for joining this thread. Allow me to share and help you with the emails.
The most possible reason why some emails from QuickBooks Online were marked as spam is that the email domain of your customer blocks the @intuit.com. When they receive your email in their spam folder, you'll want to advise them to select This is not spam or Not Junk.
Also, you'll want to inform them to send a sample email to do_not_reply@intuit.com or add quickbooks@notification.intuit.com to their contacts. This helps so that their email provider recognises the @intuit domain as a trusted site.
Need help in personalising the sales forms? Check this link: Customise invoices, quotes, and sales receipts in QuickBooks Online.
Leave a comment again below if you have other concerns. I'll be here!
