|1||The postman delivered the junk mail threw it in the trash|
|2||The tenant delivered the junk mail threw it in the trash|
|3||Postmen delivered the junk mail threw it in the trash|
|4||Tenants delivered the junk mail threw it in the trash|
Most readers find that the sentence in 3 is easier than that in 1 and that 4 is easier than 2. This is because in the first two sentences (which are reduced relative sentences equivalent to The postman/tenant who was delivered the junk mail ...) there is a background assumption being made that there is a set of postmen or tenants from which this particular postman or tenant is being selected (see discussion in the textbook of similar assumptions behind the famous and somewhat controversial garden path sentence The horse raced past the barn fell). No such assumption is made in 3 and 4.
Most readers also find that 4 and 2 are easier than 3 and 1 respectively, even though 3 and 4 have the same structure, as do 1 and 2. This is an effect of the expected role of postmen and tenants vis-à-vis delivering mail or getting mail delivered. In these sentences the correct interpretation is that the postmen or tenants are getting the mail delivered, which is more readily compatible with tenants than with postmen (who, after all, normally deliver the mail).
Overall, then, the preferred ranking of the sentences in terms of easier to harder would be 4, 2, 3, 1.