Two words that decide who owns your email: 250 OK

transcript

Two words decide who owns your email. 250 OK. Say them too early, and a message exists nowhere.

9:40 pm. Ravi emails his accountant a 19 megabyte scan. It arrives as 26 megabytes, base64 encoded. The server says 250 OK. Ravi's server may now delete its copy.

But here's the catch. What if the edge said yes too early? It crashes with the scan still only in memory. Ravi's copy is gone. Nobody has it now.

So the rule is simple. Write first. Three zones. Wait until two confirm. Only then, 250 OK. That's accepting responsibility for the message. Can't write safely? Answer 451. Try later.

Now flip it. The 250 gets lost on Ravi's network. His server retries. The queue gets the scan twice. So every delivery gets a key. Message-ID, body hash, recipient. Seen it? Say 250 again. Queue nothing.

And when your queue is drowning? Answer 451. Senders must retry, at least 30 minutes apart. For at least 4 to 5 days. Their disks become your buffer.

A temporary error costs a delay. Falling over costs mail. Say 250 only when it's safe.

Full breakdown in the lecture.