The phrase "a subsequent email" is correct and usable in written English. You can use it to describe an email that is sent at a later time than an initial email. For example, "The customer's complaints were addressed in the initial response email, but further questions were answered in a subsequent email."
A subsequent email didn't receive a reply.
A subsequent email informs me that a firm of health consultants will be coming in to support us during this period.
But a subsequent email from Lord in January said he was "extremely unimpressed" Javid had not "come up with the goods".
Asked in a subsequent email if she avoids inconvenient questions by declining to follow up, Sanders didn't follow up with an answer.
In a subsequent email, Lebouvich said he had held permanent US residency for about five years, which would mean he was entitled to contribute.
Paxman disagreed – countering by saying "it looks like we are hiding" in a subsequent email – only for Rippon to say that ducking the story looked like "the least worst option".
After BuzzFeed published a story that included photos and gifs titled, "The 33 Fiercest Moments From Beyonce's Halftime Show," they received a call and subsequent email from Bey's publicist who kindly asked them to remove the photos she deemed "unflattering". To which we have to say... huh?
A subsequent leaked email from him to Papa Massata Diack showed Davies discussing a plan to delay the announcement of positive tests by Russian athletes.
It turned out that number was inflated, and a subsequent internal email put the real number of transitioned blogs at 300,000.
Earlier today, VPN service PureVPN was dealt a double blow by malicious hackers: a zero-day exploit; and the mailing of a subsequent, fake email alleging account closure and a data compromise.
A subsequent, undated email from Wilcox suggested that Regnerus' findings be pitched to Social Science Research, where Wilcox sits on the editorial advisory board.
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MA of Applied Linguistic, Maquarie University, Australia