Essentially an alpha-2 antagonist like rauwolscine can cause an increase in serotonin levels by enhancing serotonergic neurotransmission by inhibiting receptors that decrease serotonin transmission and stimulating receptors that increase transmission. Seeing as an SSRI is already increasing serotonin levels, this can leave you with "too much serotonin" when on an SSRI, and can lead to Serotonin Symptom, which can be life threatening, but more common sides would probably be headaches, vomitting, nausea, and problems with coordination etc. (I suggest looking this up). Basically, rauwolscine has the potential to enhance the effects of your SSRI, but to an unwanted degree that can be uncomfortable and dangerous.