Fix your SMTP server records in DNS (reverse lookup not working).
Fix your SMTP server: the SMTP server you are using to verify domains does not have a reverse lookup address, so emails are either rejected or marked as spam by any server that is well configured.
I’m closing this out following a discussion with the last commenter. This was due to the recipient mail server bouncing emails. For anyone else that stumbles across this, if you reject email from HIBP then you can’t get email from HIBP! The outbound address is email@example.com
Juan Arjona commented
Troy, you can ask SendGrid (they are using aws) to ask AWS to set the reverse DNS records to their MX servers.
You can check this on Linux;
# dig haveibeenpwned.com MX
mxb.mailgun.org. 60 IN A 126.96.36.199
mxb.mailgun.org. 60 IN A 188.8.131.52
You can check the reverse lookup for any of these IPs (example):
# dig -x 184.108.40.206
which gives you: ec2-34-232-204-98.compute-1.amazonaws.com.
The answer should be the same as the initial query: mxb.mailgun.org.
It is not that servers decide to reject your unique email. Spammers normally fail to have these settings on the DNS records, as these are controlled by the network where their servers are hosted. It is common/standard protection for SMTP servers to check this and tag email as spam, or reject it.
Amazon offers their clients the ability to set these records, after checking that they are not spammers. If you fix this, you won't have emails going to the junk without you (or the recipient) knowing.
YES! I was trying to figure out why no emails were passing through
The SMTP service is run by SendGrid, are you saying there's something specific they should be doing differently? Can you point me to a reference with further info?