Format timestamp according to RFC 5322 as described in RFC 5321 (chapter 4.4):

Time-stamp-line = "Received:" FWS Stamp <CRLF>
Stamp = From-domain By-domain Opt-info [CFWS] ";"
FWS date-time
; where "date-time" is as defined in RFC 5322 [4]
; but the "obs-" forms, especially two-digit
; years, are prohibited in SMTP and MUST NOT be used.
...

envelope.go

@@ -46,7 +46,7 @@ func (env *Envelope) AddReceivedLine(peer Peer) {
 		peer.ServerName,
 		peer.Protocol,
 		tlsDetails,
-		time.Now().Format("Mon Jan 2 15:04:05 -0700 2006"),
+		time.Now().Format("Mon, 02 Jan 2006 15:04:05 -0700 (MST)"),
 	)))
 
 	env.Data = append(env.Data, line...)