How to handle mailbox full blocks

How to handle mailbox full blocks

There’s a list hygiene best practice that we think gets overlooked far too often despite the wonders it can do for your reputation. It can lower your overall block rate averages at big mailbox providers like Gmail, Microsoft, and Apple. It can remove certain would-be spam trap addresses from your list before they actually turn into spam trap addresses (blocklist prevention, anyone?). It can reduce hard bounce rate averages over time, too!

What is this magical, oft ignored best practice, you ask? It’s the practice of removing recurring blocks! While it’s not quite as easy as waving a wand, we think it’s well worth the effort.

What’s a block, and why should we care?

“Block” is SendGrid’s term for soft bounces. If you get a block response when attempting to send to a recipient, it means that something temporarily prevented the message from being delivered to an inbox. Some common reasons for a block response are:

• Recipient server was temporarily down

• Emails were deferred due to a reputation issue

• IP or domain was on a blocklist at the time of sending

• Recipient’s mailbox was full

When SendGrid receives a mailbox full type block response from a recipient server, we treat that as a temporary error. This means that we will try the address again on subsequent sends.

Even our squeaky clean senders get blocks when they send their campaigns– it’s totally normal. But blocks do demand attention and action: When there’s a blocklist issue, figure out what triggered it and how to resolve it. When emails are being deferred, slow down or even pause sending and work to figure out any needed improvements to your campaign. And when you keep getting a “mailbox full” or “user over quota” type response from a recipient, suppress that email address before it can harm your brand.

What do mailbox full/user over quota responses mean?

There are specific key phrases to watch out for you in your block responses: “mailbox full,” “over quota,” and “out of storage.” If a mailbox is full, it can mean either that the human that it belongs to is no longer engaging with email, or that the storage on the recipient address is temporarily at its limit. When mailbox providers establish that an address is inactive in this way, it will send us back these mailbox full type responses. It’s possible that the aforementioned human will come back and start engaging with emails again. But if they don’t, one of two things will happen, usually around the 12 month point:

• The email address will be turned off and you’ll receive a hard bounce response when you try mailing to it.

• A blocklist provider like Spamhaus or SpamCop or the mailbox provider itself will use the now ancient email address to be used as a spam trap address, which could cause significant damage to your reputation and ultimately your delivery rates. Once an address is turned into a spam trap, you’ll always receive a “delivered” event, which can make it harder to suss out. 

Below are some examples of block responses from some of the top B2C providers that fall under this category:

Microsoft: 

552 5.2.2 <emailexample@icloud.com>: user is over quota

Comcast:

To be very clear, the point here is not to remove or suppress every email address that gets one of these responses. Rather, pay attention to email addresses that are only getting the “mailbox full” responses over a period of time and subsequent campaigns. 

How to write block suppression logic

SendGrid’s delivery operations team recently looked at a recipient cohort of about 41 million addresses that were getting a mailbox full type of block response. Over a 30 day period, about 8.5% of those addresses went back to the active state and started seeing deliveries again. These numbers highlight how tricky finding the right mailbox full suppression logic can be: You want a system that is lenient enough to allow for active recipients to come back, but strict enough that you don’t get stuck paying for millions of emails that soft bounce into eternity.

We recommend doing your own testing to figure out numbers that make sense for your mailing lists. But there are some general best practices you can follow depending on the composition of your mailing list. 

B2C senders can and should generally allow for more mailbox full type responses simply because of the way some of these addresses behave. Recipient storage allowances at providers like Gmail and Apple now include things like docs and photos, so those addresses tend to run out of storage a lot more quickly. We have seen B2C daily senders implement suppression logic that allows for 3 to 4 consecutive mailbox full type responses with great success. If you’re a B2B sender, your tolerance for mailbox full type responses should be even lower. 

Another approach we have seen senders take is pausing sending to an address after the first mailbox full response, then trying it again a second time after 30 to 60 days. If there is a successful delivery on the second try, the address is put back into the marketing campaign flow. If there is a second mailbox full type response, the address is fully suppressed from the mailing list.

Looking for some additional guidance when it comes to building block suppression logic? Our Professional Services team is a group of email industry experts here to help you optimize your email program. Reach out and let us know how we can help!