diff --git a/imapserver/search.go b/imapserver/search.go
index bb96363..0591ae2 100644
--- a/imapserver/search.go
+++ b/imapserver/search.go
@@ -221,7 +221,9 @@ func (c *conn) cmdxSearch(isUID bool, tag, cmd string, p *parser) {
 
 		// No untagged ESEARCH response if nothing was requested. ../rfc/9051:4160
 		if len(eargs) > 0 {
-			resp := fmt.Sprintf("* ESEARCH (TAG %s)", tag)
+			// Microsoft Outlook "new" (Microsoft Office 365/15.20.7025.17) seems to fail when
+			// the tag value doesn't have double quotes. These quotes are optional.
+			resp := fmt.Sprintf(`* ESEARCH (TAG "%s")`, tag)
 			if isUID {
 				resp += " UID"
 			}