Random customers not receiving emails by invoice

We send invoice emails as pdf attachments to our customers. Typically we don't get many issues.

However, for a long time a seemingly random number of customers do not receive emails from our main account sender. The sending email address is in this format [email protected]

I can't understand what the issue could be. For example one long standing customer of 30+ years today has been late to pay, they have never paid late, but upon checking they have not received the email or the invoice by email.

To get around this we're having to send the email from our accounts person to me, and i am having to forward the email on, even though my email address is in the same format, with just a name change differentiating them.

I wonder if anyone has any suggestions here as to what the issue could be.

Should we change the format of the email generated from sage? 

Deducing the issues all i can think is that it is something to do with the email being generated automatically by sage. Perhaps there is some sort of encoding added which our customers securitt filters do not like. If we then forward the email on (from the sent items folder) it is typically received.

Hopefully someone can offer some suggestions here.

Thanks