My Experience With Yahoo's TSS04 Error

Recently, I started receiving a temporary deferral from Yahoo indicating that emails were being delayed due to unexpected volume or user complaints.

[TSS04] Messages from X.X.X.X temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

In my experience, this error often occurs when sending a large email volume from a new IP address. Yahoo simply doesn’t expect high traffic from it. That wasn’t the case here, though. This IP had been in use for around six months, and I hadn’t encountered these error codes before.

Surely, something must be wrong, right?

First, I had to check if the whole /24 was blacklisted or not. So what I did was onboarded a few more IP addresses for the same range, and luckily, Yahoo accepted the emails without any issues. So I ruled out any issues with the complete IP block.

After that, I investigated the users on that range and I did find a bunch of customers, but found nothing suspicious or malicious in the email content, with nearly all emails being marketing in nature. However, something must have triggered Yahoo to reject these emails.

I reached out to Yahoo Postmaster, but the response was rather unhelpful. Nonetheless, they did ask me to wait for "two weeks" which possibly indicates that their internal systems need time to reset or clear any rate limits tied to the IP address.

That said, I disabled the IP addresses in question and dropped all existing queues. I did send a few emails occasionally, but these were throttled as well. Most people on the web mention that usually waiting a few hours is enough, but in our case, not really.

Furthermore, after correlating some data from Google's Postmaster Tools, I managed to pinpoint the customer responsible for the issue. Although their email content was OK, the email list they were using probably purchased or mined, which resulted in their emails being marked as spam or having a poor interaction rate.

Finally, after three days of dead silence, the emails coming from these IP addresses were once again accepted by Yahoo without any deferrals.

So if you're having troubles with TSS04, here are my recommendations:

• Check what you're sending. Do people really want these email addresses? Did they sign up for it?
• Have you done an email warmup before sending bulk emails?
• Sometimes, you just gotta slowdown, cowboy. Temporarily disable these IP addresses.

If your email list is opt-in and you're still seeing this issue, then you definitely need a larger IP pool and cull down unengaged subscribers' email addresses.

Areeb Majeed is the co-founder of Maileroo. With a background in technology, Areeb plays a key role in shaping the company's technical strategy and overseeing its implementation. This article was originally published on: https://areeb.com/blog/troubleshooting-yahoo-tss04