The Tandarans were at war with the Suliban Cabal since 2144. Shortly after the war began, the Tandarans began questioning the loyalty of the unallied Suliban living in the Tandar sector. They were relocated to places such as Detention Complex 26 "temporarily." Years later, they were still waiting to be released. (ENT: "Detained")
In February 2152, Jonathan Archer encountered a physically altered Tandaran agent named Keyla on Risa. He noted that her surgeons "did a nice job," adding that "[a]ll the Tandarans I met had a very distinctive feature, right here [on the bridge of the nose]." (ENT: "Two Days and Two Nights")
In the final draft script of "Detained", the Tandarans were introduced with the description, "Aside from a distinct cranial pattern, they closely resemble humans." The Tandaran guards in Detention Complex 26 were also scripted to wear uniforms.
It seems likely this species was originally called Mazarite. Initial reports from the making of the installment refer to them as such.  In addition, a page from the "Detained" script referred to one of the Tandarans as a "Marazite" even on the same date as the name was finalized, suggesting this was a typo (which may explain why it wasn't removed). The name Mazarite was later used for a different species in ENT: "Fallen Hero".
In dialogue from an almost entirely rewritten scene from the script of "Detained", T'Pol disingenuously told Colonel Grat, "I've been doing some research on your species. You appear to be a gregarious, amiable race, that disapproves of violence." Later in the same scene, she asked him if the Tandaran government was prepared to answer for the deaths of the Enterprise crew members and finally concluded, "If I may be so bold, Colonel... I believe it's logical to assume that, one day, your world and Earth will establish formal relations."