Tesla's shares rose more than 6 percent to hit $1,000 on Wednesday after Reuters reported Musk's email, making the Silicon Valley company the second-most valuable automaker in the world, behind Toyota. Tesla's share surge could position Musk for another payout from a stock compensation plan tied to the company's market capitalization.
The company's shares closed Wednesday's trading up nearly 9 percent to $1,025.05.
The stock also got a boost from a bullish call by Wedbush, which lifted its target price for the automaker to $1,000 and said Tesla could have "more room to run."
Investors also are anticipating Musk will unveil new "million-mile" battery technology that could deliver longer life, lower costs and better range for future Tesla vehicles.
"Production of the battery and powertrain will take place at Giga Nevada," Musk wrote about the Semi. Most of the other work will probably take place in other states, he wrote, without stating where.
Musk has said Tesla is scouting other U.S. states for a site to build a new factory, hinting that Texas could be a candidate. Oklahoma and other states are campaigning for the investment.
When Musk unveiled the prototype of the futuristic, battery-powered Semi in 2017, he said the Class 8 truck would go into production by 2019. More recently, he said the Semi would go into volume production by 2021.
Musk's message coincides with a surge in the share price of rival electric and fuel cell truck maker Nikola.
Nikola, a startup, began trading on the NASDAQ this month after it merged with special purpose acquisition company VectoIQ.
Shares in Nikola have more than doubled in price over the past week as the company's CEO has used Twitter and interviews to promote plans to launch an electric pickup truck.
Nikola and CNH Industrial's IVECO commercial truck operation last year formed a joint venture to build a battery electric and fuel cell truck line called the Nikola Tre. IVECO has said orders are strong for the electric version of the truck, due out next year.
Nikola on Wednesday said it had hired a former Tesla executive, Mark Duchesne, to lead its manufacturing and a former Caterpillar executive, Pablo Koziner, to head its hydrogen fueling and battery recharging business.