I think it depends on whom the email is going to. If you're sending emails to people for whom a Master-level MOS is meaningful, then you could include it; at worst, if you include it in all emails, it may be a conversation starter.I have a correspondent
who signs all emails with his full business signature, which includes the letters "RCDD, RTPM." Mean anything to you? Me either, but I did google them; they're still pretty much meaningless to me, but I know they mean he has achieved distinction or credentials in
his field.
I have a client with a string that includes Ph.D., CPA/ABV/CFF, MAI, SREA, SRA, ASA, CRE, and FRICS. Every letter or report he generates includes, in the letterhead or signature block, a subset of these designations designed to impress the intended reader.