Short answer:
Create a .Rhtml file for the email body.
Knit it and read:
knitr::knit("tale_email_body.Rhtml")
library("readr", lib.loc="~/R/win-library/3.2")
eb <- read_lines("tale_email_body.html",n_max= -1L)
eb2<-paste(eb, sep="", collapse="")
Use the results in the body of the letter:
library(RDCOMClient)
olMailItem <- 0
OutApp <- COMCreate("Outlook.Application")
outMail = OutApp$CreateItem(olMailItem)
outMail$GetInspector()
signature <- outMail[["HTMLBody"]]
outMail[["To"]] <- sm
outMail[["CC"]] <- paste("egrp",dm,sep=";")
outMail[["subject"]] <- "note this"
outMail[["BodyFormat"]] <- 2
outMail[["HTMLbody"]] <- paste0(eb2, signature)
outMail$Display()
outMail$Send()
Let me know if you have any questions or improvements.
Partial Credit: How to add my Outlook email signature to a COM object using RDCOMClient