There are not many Christian tribes mentioned. They were mostly Jewish. Many of Mohammad's wives were from Jewish tribes, except for
Maria bint Shamʿūn (Maria al-Qibtiyya) who was an Egyptian Coptic slave (her status as Mohammad's wife is not clear).
Things like this are why another poster here keeps making threads about non-Muslims who are ardent hadith believers.
This is all just hadith/sirat literature which non-Muslims should probably treat with a fair degree of scepticism given the overall content
The idea he was illiterate and came from a pagan backwater is a bit like the virgin birth: a miracle that attests to his status.
Note that the early Muslims supposedly fled to Christian Axum (Ethiopia), a Roman aligned local power who had been active in wars against Persian aligned Jewish tribes in the Arabian Peninsular in the century leading up to Muhammad's birth.
There is a reasonable chance 'Muhammad's people' were one of the Roman aligned tribes hired as mercenaries or operating as suppliers to the empire.
except for
Maria bint Shamʿūn (Maria al-Qibtiyya) who was an Egyptian Coptic slave (her status as Mohammad's wife is not clear).
Her status as anything to do with Muhammad is not clear given the story of how she came to be Muhammad's wife is rather fantastical.
If they have a historical root, the story of the letters to Mukawkis likely relate to the later communication between ibn al-As and The Patriarch Cyrus. There are multiple independent sources that acknowledge the communication between Cyrus and ibn al-As. A marriage alliance was also proposed between the general and a Byzantine Princess (assuming he converted of course).
On the other hand, Cyrus wasn't even in Egypt at the right time to communicate with Muhammed, and the idea he sent some Christian slaves in tribute to a 'heretic' prophet of minor importance is quite dubious.
She could be real, but 'acquired' in a different way.
It could be an telescoping of later factual info about someone into the prophetic biography.
It could just be a trope of a powerful 'enemy' showing respect to Muhammad thus attesting to his special status (see the other letters he supposedly wrote. The story about Heraclius is obviously a total fabrication).