Orsino sees Viola's feminie allure even through her disguise. Thus, he believes her to be the perfect messenger to send to Olivia.
"Dear lad, believe it.
For they shall yet belie thy happy years
That say thou art a man. Diana’s lip
Is not more smooth and rubious. Thy small pipe
Is as the maiden’s organ, shrill and sound,
And all is semblative a woman’s part.
I know thy constellation is right apt
For this affair."
Twelfth Night/ Act I/ Scene IV