You need to add IFluentMail.ReplyTo method invocation and specify email address as a parameter:
.ReplyTo(new MailBox(emailReplyTo))
Here's the full code:
Limilabs.Mail.Fluent.Mail.Html(Template
.FromFile("EmailTemplate.txt")
.DataFrom(templateData)
.Render())
.From(new MailBox("myemail@email.com", "Me"))
.To(new MailBox(emailTo))
.ReplyTo(new MailBox(emailReplyTo)) // <---
.Subject("My Subject Line")
.UsingNewSmtp()
.WithCredentials("myemail@email.com", "password")")
.Server("smtp.gmail.com")
.WithSSL()
.Send();