recipients/clients not receiving emails

recipients/clients not receiving emails

Every now and then recipients of our emails, mostly our clients, do not receive the emails we have sent them for a period of time - for days at a time, sometimes weeks.

Unfortunately we do not get any messages stating they have not received them, and therefore we assume the emails have gone through OK. It is only later on that we realise they have not and we try to pin point for how long this has been occurring.

Most emails do not have any file attachments to them, so they are very small in size.

We do not have any problems receiving emails from them.

This is most frustrating, as you can imagine, as it is detrimental to our work.

Any ideas what is happening here?

Thanks
M