The only way that size could be a problem
Is if the Poster doesn't clear their Server Mailbox regularly. If their ISP's Mail Box is close to full it will not accept a incoming E Mail that pushes the Mail Box over the Limit which with most ISP's is 10 Meg.
So I suppose here the question that needs asking is how often is the Incoming E Mail collected from the ISP? If that is the case here the poster needs to collect their mail more often.
Col