add "RcptTo" to webapi MessageGet result

otherwise, if the recipient was a bcc, there's no good way to see why the
message was received.

incoming webhooks already have this rcptto field, but that's not always the
moment you want to process it.

for mattanja on matrix, thanks for reporting!

webapisrv/server.go

@@ -1245,12 +1245,17 @@ func (s server) MessageGet(ctx context.Context, req webapi.MessageGetRequest) (r
 	if d, err := dns.ParseDomain(m.MsgFromDomain); err == nil {
 		msgFrom = smtp.NewAddress(m.MsgFromLocalpart, d).Pack(true)
 	}
+	var rcptTo string
+	if m.RcptToDomain != "" {
+		rcptTo = m.RcptToLocalpart.String() + "@" + m.RcptToDomain
+	}
 	meta := webapi.MessageMeta{
 		Size:                m.Size,
 		DSN:                 m.DSN,
 		Flags:               append(m.Flags.Strings(), m.Keywords...),
 		MailFrom:            m.MailFrom,
 		MailFromValidated:   m.MailFromValidated,
+		RcptTo:              rcptTo,
 		MsgFrom:             msgFrom,
 		MsgFromValidated:    m.MsgFromValidated,
 		DKIMVerifiedDomains: m.DKIMDomains,