cannot see all contact in a group email (To:)

I have EM Client installed across several machines in my workshop, and one of them has stopped showing most of the contacts in a group email (To:).

Basically when I read an email which has been sent to several people (e.g. To: Jack@, Peter@, Tom@), I only see my name Jack@. Where as on my other computers I can see all of the contacts in the list.

I’ve checked amongst the options and cannot find anything which leans towards fixing this?
Does anyone have a thought as to what might be causing this issue?

Thanks,
Jack.

Is the email account on which the other addresses are not visible using a different email server to the ones where they are visible?