Once sperm cells are mature and capable of fertilizing an egg they are stored for delivery in this part of the male reproductive system:

The answer is epididymis.
Epididymis is a tightly coiled mass of thin tubes that carries sperm from the testes to the ductus deferens in the ale reproductive system. Sperms matures as they pass through the epididymis so that they are ready to fertilize ova by the time they enter the ductus deferens.  During the ejaculation stage of emission sperms are moved from the testes and the epididymis, where they are stored, to the beginning of urethra.