Yahoo servers receiving PDF invoices
Recently I had a customer issue where their clients could not receive PDF attached invoices that they were sending.
If the recipient tried downloading or previewing the email, it wouldn't appear in the preview window, or it would say the attachment may contain a virus.
I scanned the domain computers and the servers. I asked the client to send me a copy of the invoice and i could receive it and preview it OK. It just seemed to happen with some email addresses.
As the clients emails were hosted with G suite (Google Emails), I contacted Google and gave them the error details. I then accessed the clients DNS records and added the new SPF and DKIM records. After they were setup it helped authenticated the clients emails when sending out from their domain. Once the records had propagated (registered with the Internet so all servers could see the records when requesting), emails servers we were sending the invoices through could see that the emails were coming from a legitimate source and were then passing the emails, and more importantly, the invoices on to the recipient and their email client (like Outlook installed on their pc).
Now; there still is a problem with Yahoo servers. For some reason, they think the PDF is corrupted and is marking the PDF as a false positive virus attachment. Usually we don't get larger or enterprise clients hosting their email accounts with Yahoo, so it isn't affecting too many accounts. My recommendation is to update the client records and their email address to another email account until this issue is resolved.
This is affecting business accounting software packages like Sage Accounting also. Yahoo servers are marking the PDF attachment as virus attached if there is lines of code in the header that it doesn't expect. Some PDF printing components found in the accounting software are adding in extra lines of blank code (ten lines) in the bottom of the invoice. When servers like Yahoo find the corruption, they are marking it as a virus attached.