We've been experiencing the issue for a while now that customers aren't receiving their invoices and packing slips when we send them via TradeGecko's emailing system. The e-mail addresses are correct (when they're not, we do correctly read that the e-mail was 'dropped', and they often receve them when we resend a few days later), and we ask them to check spam.
The history even reads that the email was 'delivered' and 'opened'. Has anyone else had this issue? How can we resolve it?