The Sages debate whether the Torah’s use of the plural “doorposts” indicates that a doorway with only one post does not require a mezuzah. In practice, if there is a post on the right side, we affix a mezuzah, without a berachah.[1] Here are some other typical cases of single doorposts which are topped by lintels:
If a doorway has one doorpost on the right side of the person entering the room, and on its left, the wall continues without a break, a mezuzah should be affixed on the right.[2] This is true even if that doorpost is nothing more than the end of a wall.[3][A]
If there is a doorpost on the right, but no wall continuing on the left, no mezuzah is necessary.[4]
If the sole doorpost is on the left, and the continuing wall is on the right, no mezuzah is necessary.[B]
However, if the right wall continues less than three tefachim of the left wall, they are deemed to be in alignment enough to require a mezuzah on the right post. Even so, no berachah is recited. [5][C]
[1] Shach Y.D. 287:1.
[2] Aruch HaShulchan 247:8; Shevet HaLevi 5:160; Agur B’ohalecha )20:8( cites Maharsham )4:111(, who rules that if there is no door, one may exempt this opening in a case of need.
[3] Agur B’ohalecha )20:10 and 23:5( rules that one may exempt this opening in a case of need.
[4] Aruch HaShulchan 287:8; Shevet HaLevi 5:160; Agur B’ohalecha 20:8.
[5] Da’as Kedoshim cited in Chovas HaDar 7 and Mezuzot Beitecha 287:1.