diff --git a/MailKit/Net/Imap/ImapCommand.cs b/MailKit/Net/Imap/ImapCommand.cs
index fd57412955..d819c5c3e4 100644
--- a/MailKit/Net/Imap/ImapCommand.cs
+++ b/MailKit/Net/Imap/ImapCommand.cs
@@ -682,7 +682,7 @@ void UpdateProgress (int n)
 
 		static bool IsAtom (char c)
 		{
-			return c < 128 && !char.IsControl (c) && "(){ \t%*\\\"]".IndexOf (c) == -1;
+			return c < 128 && !char.IsControl (c) && "(){ %*\\\"]".IndexOf (c) == -1;
 		}
 
 		static bool IsQuotedSafe (ImapEngine engine, char c)