Private letters in the Roman period typically have the letters text on one side of the papyrus only; the back is kept for the address, typically a single line down the back at right-angles to the text on the front. This was because the practice was to fold the letter into a thin spill, which would be tied around with a fibre of papyrus and sealed with a small clay seal; the address typically simply in the form To X from Y would be written down one of the sides left exposed (generally the side with the seal), which are usually much dirtier than the protected inside surfaces. Only rarely are house details given as part of the address. Except for those on government and military service, there was no organised postal service; letter-writers normally would have to rely on delivery by a carrier who knew where to find the addressee. A letter might never reach its destination.
A private letter, circa AD 217-8
The back of this letter is exhibited for its unusually elaborate address, written in two panels which perhaps occupied both sides of the spill into which the letter had been folded for delivery.
When the letter had been folded, it was tied and then sealed with a clay seal through which the tie would run. For added security, ink marks were then made running across the top of the seal; this is why the names of sender and addressee on letter backs are frequently separated by a large X with its centre missing, that being the point where the seal had been affixed. The ink marks here are clumsy but there seems to be a blank patch as expected. The upper ladder may be the remains of lines that were drawn across the tie on the other side of the spill.
The address translates: From Sarapas and Gaia, and then in the lower panel To Eutyches who distributes wreaths under the gateway of the Serapeum by the great image. Added almost as a postscript at upper right: Give (the letter) to Ammonius who distributes wreaths at the shrine of the god, and he will give it to him.
The Oxyrhynchus Papyri vol.XLIII 3094