[FIX] refactor error messages and FQDN regex usage

Author: valentin-kaiser

mail/internal/email/email.go

@@ -127,13 +127,13 @@ func NewFromReader(r io.Reader) (*Email, error) {
 
 		contentType, _, err = mime.ParseMediaType(p.header.Get("Content-Type"))
 		if err != nil {
-			return msg, apperror.NewErrorf("Could not parse Content-Type header with value %q", p.header.Get("Content-Type")).AddError(err)
+			return msg, apperror.NewErrorf("could not parse Content-Type header with value %q", p.header.Get("Content-Type")).AddError(err)
 		}
 
 		if cd := p.header.Get("Content-Disposition"); cd != "" {
 			cd, params, err := mime.ParseMediaType(p.header.Get("Content-Disposition"))
 			if err != nil {
-				return msg, apperror.NewErrorf("Could not parse Content-Disposition header with value %q", p.header.Get("Content-Disposition")).AddError(err)
+				return msg, apperror.NewErrorf("could not parse Content-Disposition header with value %q", p.header.Get("Content-Disposition")).AddError(err)
 			}
 			filename, filenameDefined := params["filename"]
 			if cd == "attachment" || (cd == "inline" && filenameDefined) {
@@ -728,7 +728,7 @@ func parseMIMEParts(hs textproto.MIMEHeader, b io.Reader) ([]*part, error) {
 	}
 	ct, params, err := mime.ParseMediaType(hs.Get("Content-Type"))
 	if err != nil {
-		return ps, apperror.NewErrorf("Could not parse Content-Type header with value %q", hs.Get("Content-Type")).AddError(err)
+		return ps, apperror.NewErrorf("could not parse Content-Type header with value %q", hs.Get("Content-Type")).AddError(err)
 	}
 	if strings.HasPrefix(ct, "multipart/") {
 		if _, ok := params["boundary"]; !ok {
@@ -749,7 +749,7 @@ func parseMIMEParts(hs textproto.MIMEHeader, b io.Reader) ([]*part, error) {
 			}
 			subct, _, err := mime.ParseMediaType(p.Header.Get("Content-Type"))
 			if err != nil {
-				return ps, apperror.NewErrorf("Could not parse Content-Type header with value %q", p.Header.Get("Content-Type")).AddError(err)
+				return ps, apperror.NewErrorf("could not parse Content-Type header with value %q", p.Header.Get("Content-Type")).AddError(err)
 			}
 			if strings.HasPrefix(subct, "multipart/") {
 				sps, err := parseMIMEParts(p.Header, p)

mail/security.go

@@ -12,7 +12,11 @@ import (
 	"golang.org/x/time/rate"
 )
 
-var fqdnPattern = regexp.MustCompile(`^([a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{1,}$`)
+// FQDNPattern validates a fully qualified domain name (FQDN) with the following rules:
+// - One or more labels separated by dots.
+// - Each label is 1-63 characters: alphanumeric and hyphens, but cannot start or end with a hyphen.
+// - The final label (TLD) must be alphabetic and at least one character.
+var FQDNPattern = regexp.MustCompile(`^([a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?\.)+[a-zA-Z]{1,}$`)
 
 // SecurityManager handles security validations and tracking for both SMTP client and server
 type SecurityManager struct {
@@ -368,7 +372,7 @@ func (sm *SecurityManager) isValidFQDN(hostname string) bool {
 	}
 
 	// Validate format using regex
-	return fqdnPattern.MatchString(hostname)
+	return FQDNPattern.MatchString(hostname)
 }
 
 // cleanup periodically cleans up old entries