Why can't I send email from Outlook (microsoft account) to a Yahoo or AT&T email account?

They're both right and wrong at the same time because neither Yahoo nor Microsoft can edit your DNS records. You (the domain owner) are the one responsible for fixing your DNS records. Unfortunately the majority of users have no clue what I'm talking about. They wait for something that will never be fixed unless they take action (add correct SPF/DMARC/DKIM records to their DNS records).

If you look at the bounced emails, you'll see that Yahoo disconnects the socket rather than actively rejecting the email. That forces the email into Microsoft's retry queue. I've seen bounced messages that retried 11 times. It makes no sense that Yahoo would increase traffic like that rather than actively refusing the email with an error - IF it's a DKIM/DMARC issue.

Also, according to Yahoo, regular email senders require only an SPF OR DKIM. It's clearly stated in their official Sender Requirements. When a Microsoft account is first set up, they provide the necessary DNS records including the SPF. I'd bet that 99.99% of the Microsoft custom domain users have proper SPF records.

If Yahoo now requires SPF AND DKIM, they should update their published requirements. If it's still not their policy, I'd say there's something wrong at Yahoo.

>> I'd bet that 99.99% of the Microsoft custom domain users have proper SPF records.

Many don't have an SPF record or multiple SPF records (this is a more common problem especially with GoDaddy customers). That's what I notice.

The SPF and DKIM requirements are for bulk senders. We don't know who those people (who are having bouncing email problem) are. I would create all 3 records (SPF, DMARC and DKIM) to save the headache. I have all 3 and Yahoo does not reject my email (@custom domain)

https://senders.yahooinc.com/best-practices/?guccounter=1

Update

I just sent a test mail from my custom domain to my yahoo.com address. It is graylisted. I didn't have any problem, ever. A test mail arrived safely last week within a few second after sending it.

There is clearly something going on between Microsoft and Yahoo mail servers. No one knows what it is.

This is what I see in the message tracking log:

Reason: [{LED=450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError)};{MSG=};{FQDN=mta7.am0.yahoodns.net};{IP=67.195.228.109};{LRT=3/11/2024 9:18:56 PM}]. OutboundProxyTargetIP: 67.195.228.109. OutboundProxyTargetHostName: mta7.am0.yahoodns.net

That IP address belongs to Microsoft and not blacklisted as of the time I sent the email.

My test email finally arrived 11 minutes later. It should've been instant delivery (within a few seconds).

At this point there may be multiple issues between Microsoft SMTP servers and/or IP addresses and Yahoo's.

Setting up a Microsoft account won't complete the Domain configuration until it sees all of the DNS records they require configured on the custom domain. I'll have to stick by my 99.99% number.

Virtually every client with a custom domain I have on Microsoft O365 is experienced the problem. The SPF is a requirement only when you're enlisting a third party to send email on your domain's behalf. My clients aren't bulk emailing. According to Yahoo, the SPF should be sufficient for non-bulk senders.

What it looks like to me is that Yahoo's SPF lookup is failing. I can set just DKIM and email goes through. But that still doesn't explain why Yahoo doesn't actively return an error to Microsoft instead of just disconnecting.

I agree. Adding DKIM and DMARC along with SPF is no longer optional.