diff --git a/src/libraries/System.Net.Mail/src/System/Net/Mime/MimeBasePart.cs b/src/libraries/System.Net.Mail/src/System/Net/Mime/MimeBasePart.cs
index e0776147cb11c4..35a642ee56667e 100644
--- a/src/libraries/System.Net.Mail/src/System/Net/Mime/MimeBasePart.cs
+++ b/src/libraries/System.Net.Mail/src/System/Net/Mime/MimeBasePart.cs
@@ -10,7 +10,6 @@ namespace System.Net.Mime
     internal class MimeBasePart
     {
         internal const string DefaultCharSet = "utf-8";
-        private static readonly char[] s_decodeEncodingSplitChars = new char[] { '?', '\r', '\n' };
 
         protected ContentType? _contentType;
         protected ContentDisposition? _contentDisposition;
@@ -96,14 +95,16 @@ internal static string DecodeHeaderValue(string? value)
                 return null;
             }
 
-            string[] subStrings = value.Split(s_decodeEncodingSplitChars);
-            if ((subStrings.Length < 5 || subStrings[0] != "=" || subStrings[4] != "="))
+            ReadOnlySpan<char> valueSpan = value;
+            Span<Range> subStrings = stackalloc Range[6];
+            if (valueSpan.SplitAny(subStrings, "?\r\n") < 5 ||
+                valueSpan[subStrings[0]] is not "=" ||
+                valueSpan[subStrings[4]] is not "=")
             {
                 return null;
             }
 
-            string charSet = subStrings[1];
-            return Encoding.GetEncoding(charSet);
+            return Encoding.GetEncoding(value[subStrings[1]]);
         }
 
         internal static bool IsAscii(string value, bool permitCROrLF)