fix: Correct message parsing for "receiving at a rate" error (#462)

Author: amaury1093

core/src/smtp/mod.rs

@@ -238,7 +238,7 @@ async fn email_deliverable(
 			// emails.
 			// 4.2.1 The user you are trying to contact is receiving mail at a rate that
 			if err_string
-				.contains("The user you are trying to contact is receiving mail at a rate that")
+				.contains("the user you are trying to contact is receiving mail at a rate that")
 			{
 				return Ok(Deliverability {
 					has_full_inbox: false,
@@ -278,8 +278,6 @@ async fn email_deliverable(
 				// 550 5.1.1 No such user - pp
 				// 550 No such user here
 				|| err_string.contains("no such user")
-				// 4.2.1 The user you are trying to contact is receiving mail at a rate...
-				|| err_string.contains("is receiving mail at a rate")
 				// 550 5.1.1 : Mailbox not found
 				// 550 Unknown address error ‘MAILBOX NOT FOUND’
 				|| err_string.contains("not found")