Fix ToFromString to handle and trim empty addresses

Previously, the ToFromString function split email addresses by commas but did not handle empty addresses or trim whitespace. Now, it trims each address and ignores any empty entries to ensure only valid addresses are processed. This prevents potential errors stemming from malformed input.

msg.go

@@ -753,7 +753,16 @@ func (m *Msg) ToIgnoreInvalid(rcpts ...string) {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.3
 func (m *Msg) ToFromString(rcpts string) error {
-	return m.To(strings.Split(rcpts, ",")...)
+	src := strings.Split(rcpts, ",")
+	var dst []string
+	for _, address := range src {
+		address = strings.TrimSpace(address)
+		if address == "" {
+			continue
+		}
+		dst = append(dst, address)
+	}
+	return m.To(dst...)
 }
 
 // Cc sets one or more "CC" (carbon copy) addresses in the mail body for the Msg.