Initialize address header map and enforce single 'From' address.

This commit ensures that the address header map is properly initialized before assigning addresses to it. Additionally, it enforces the rule that only a single 'From' address is allowed, preventing multiple addresses from being set for the 'From' header. These misses were found while working on the enhanced testing suite

msg.go

@@ -583,6 +583,9 @@ func (m *Msg) SetAddrHeader(header AddrHeader, values ...string) error {
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc5322#section-3.4
 func (m *Msg) SetAddrHeaderIgnoreInvalid(header AddrHeader, values ...string) {
+	if m.addrHeader == nil {
+		m.addrHeader = make(map[AddrHeader][]*mail.Address)
+	}
 	var addresses []*mail.Address
 	for _, addrVal := range values {
 		address, err := mail.ParseAddress(m.encodeString(addrVal))
@@ -591,7 +594,14 @@ func (m *Msg) SetAddrHeaderIgnoreInvalid(header AddrHeader, values ...string) {
 		}
 		addresses = append(addresses, address)
 	}
-	m.addrHeader[header] = addresses
+	switch header {
+	case HeaderFrom:
+		if len(addresses) > 0 {
+			m.addrHeader[header] = []*mail.Address{addresses[0]}
+		}
+	default:
+		m.addrHeader[header] = addresses
+	}
 }
 
 // EnvelopeFrom sets the envelope from address for the Msg.