Hi @bbar,
In my experience, this can be fixed almost always when you set up an SMTP plugin. You can set up SMTP on WordPress using a plugin, there are several free options available, but first I recommend you watch this video tutorial for more details: How To Setup WordPress SMTP For Sending Emails.
You can configure the SMTP plugin with your own email server, if your hosting provider includes one, or using a third-party transactional service. But please note that some of them has limitations, like Brevo (formerly Sendinblue) which only support 1 attachment per message, that is, if you have the Professional extension and attach a second document, e.g. the credit note, one of them will be skipped with this email service. Although if you are 100% that you will always attach only 1 document, you can ignore this warning.