I have a FMP invoicing solution that once a day emails out attached PDF invoice copies to our customers. Most days its 20-40 individual emails with an attached PDF (about 200k size). I am getting comments from some customers telling me that they don't get the invoice emails. I get BCC copies of all these emails so I know that they got sent. We also send emails acknowledgements, order ready automated emails with no issues, and statements at the end of each month. Is there anything that I should or shouldn't be doing when sending these emails so they don't get blocked or put into spam?
I currently have the FMP emails going through our ISP's SMTP mail service or should I be sending it through our normal email service (Microsoft Office 365) provider?
Would it be better to use one of the email plugins and/or use one of the API services with a web email service?