Are open rates useless now?

A reply rate that falls inside a week is almost always a change in the list, the mailbox, or the offer. The wording is the last place to look, not the first.

A reply rate that falls inside a week is almost always a change in the list, the mailbox, or the offer. The wording is the last place to look, not the first. Compare the fortnight before the drop with the fortnight after, on the same mailbox and the same offer, and see which of those three moved.

Look at the last two weeks, not the copy deck

Pull the sends from the fortnight before the drop and the fortnight after. Same offer, same list source, same mailbox, or not. A reply rate moves when one of those three changes. Rewriting subject lines before you know which one moved is how a team spends a week on the wrong job.

If the list got wider, the new names are the first place to look. A bought file, a looser job title, or a second country will pull the rate down even when the email itself is unchanged. Split the report by the week the new rows were added.

What to check before you rewrite

Do these in order. The later ones are a waste of time if an earlier one is already true.

If the first two are clean, the wording is finally the right job. Change one thing, on a slice small enough that you can read every reply, and leave the rest of the send alone until that slice has an answer.

1. Same offer, same list source, same mailbox, or a change in one of them during the fortnight.
2. Bounces and complaints on that mailbox, and whether Gmail or Outlook started deferring.
3. A small send to people you already have a reason to write to, from the same mailbox.
4. Only then, the first line and the ask, on a slice small enough that you can read the replies.

Then check the mailbox

A sudden fall that hits every campaign on the same domain is rarely a wording problem. Look at bounces, complaints, and whether Gmail or Outlook started deferring. Google's sender guidelines are the reference for bulk mail to Gmail: authentication, a spam rate under 0.10%, and one-click unsubscribe. A forum thread is not a target rate for your list.

Send a small test from the same mailbox to people you already have a reason to write to. If those still get replies and the cold slice does not, the domain is probably fine and the list or the offer is not. If the small test dies too, stop. A mailbox that cannot reach people who already know you will not reach strangers, and a new subject line will not change that.

Write the comparison down

A rate with no denominator is a feeling. Count the sends, the replies you would be glad to receive, and the week they landed. Split that by list source and by mailbox. The drop is only real when the same slice is quieter than it was.

If one mailbox fell and the others held, leave the copy alone and look at that mailbox. If every mailbox fell on the day you added a new file, the file is the lead. If nothing in the send changed and the complaints rose, stop and read the bounce and spam reports before you write another line.

Then decide what you will not change

Write down the one cause you are testing, and leave the rest of the send as it was. A new subject line, a new list, and a new mailbox in the same week will move the rate, and you will not know which one did it.

Read the replies you do get. A shorter reply, a ruder reply, or a reply that asks you to stop is still information. Count the ones you would be glad to receive, and keep that count next to the sends, for the same fortnight.