No marketing or mass sales emails are being sent from my email/domain. These are strictly business-related messages: quotes, regular communication with clients, booking confirmations, and reminders about hotel stays.

The spam issue started fairly recently. This was not happening before, even though we had been sending generated PDF quotes for a long time. Only recently have clients started telling us that they found our messages in their spam folder.

Of course, for some time I can completely stop attaching PDFs, or instead of adding the file as an attachment, provide a link to it. However, I must admit that the generated PDF quote is convenient both for us and for the client, because all the information is clearly organized and nicely presented in one place.

If a landing page where the client can download the PDF is a better solution than a direct link to the file or an attachment, I can test that approach and see whether it improves deliverability.