Update smtpd.go: Adding case-insensitive flags to regex'es

The syntax is taken from https://stackoverflow.com/a/15326471/1668200
Do you have tests for case-insensitivity?

internal/smtpd/smtpd.go

@@ -27,8 +27,8 @@ import (
 var (
 	// Debug `true` enables verbose logging.
 	Debug      = false
-	rcptToRE   = regexp.MustCompile(`[Tt][Oo]: ?<([^<>\v]+)>( |$)(.*)?`)
-	mailFromRE = regexp.MustCompile(`[Ff][Rr][Oo][Mm]: ?<(|[^<>\v]+)>( |$)(.*)?`) // Delivery Status Notifications are sent with "MAIL FROM:<>"
+	rcptToRE   = regexp.MustCompile(`(?i)TO: ?<([^<>\v]+)>( |$)(.*)?`)
+	mailFromRE = regexp.MustCompile(`(?i)FROM: ?<(|[^<>\v]+)>( |$)(.*)?`) // Delivery Status Notifications are sent with "MAIL FROM:<>"
 
 	// extract mail size from 'MAIL FROM' parameter
 	mailFromSizeRE = regexp.MustCompile(`(?U)(^| |,)[Ss][Ii][Zz][Ee]=(.*)($|,| )`)