emails are bouncing back when sending to yahoo

You need to have SPF, DKIM and DMARC all in place and correct. I'll past the relevant part below. If you have an Office 365 account, the sender is not you, it's Microsoft.com, so the second part ("more than 5,000 messages per day") applies:

See https://www.proofpoint.com/us/blog/email-and-cloud-threats/google-and-yahoo-set-new-email-authentication-requirements

The new requirements are broken down into two categories. All senders will need to follow the first set. Depending on how much email you send per day, there are also additional rules. Applicable to all senders:

1. Email authentication. This is a critical measure to help prevent threat actors from sending email under the pretense of being from your organization. This tactic is referred to as domain spoofing and, if left unprotected, allows cyber criminals to weaponize sending domains for malicious cyber attacks.
   • SPF is an email authentication protocol designed to prevent email spoofing, a common technique used in phishing attacks and email spam. As an integral part of email cybersecurity, SPF enables the receiving mail server to check whether incoming email comes from an IP address authorized by that domain’s administrator.
   • DKIM is a protocol that allows an organization to take responsibility for transmitting a message by signing it in a way that mailbox providers can verify. DKIM record verification is made possible through cryptographic authentication.
2. Low spam rates. If recipients report your messages as SPAM at a rate that exceeds the new .3% requirement, your messages could be blocked or sent directly to a spam Folder.

Requirements for senders of more than 5,000 messages per day:

1. SPF and DKIM must be in place. Companies that send to Gmail or Yahoo must have Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) authentication methods implemented.
2. Companies must have a DMARC policy in place. DMARC, which stands for Domain-based Message Authentication, Reporting and Conformance, is an email authentication standard that provides domain-level protection of the email channel.
   • DMARC authentication detects and prevents email spoofing techniques used in phishing, business email compromise (BEC) and other email-based attacks.
   • DMARC builds on the existing standards of Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM). It is the first and only widely deployed technology that can make the header “from” domain trustworthy. The domain owner can publish a DMARC record in the Domain Name System (DNS) and create a policy to tell receivers what to do with emails that fail authentication.
3. Messages must pass DMARC alignment. This means that the sending Envelope From domain is the same as the Header From domain, or that the DKIM domain is the same as the Header From domain.
4. Messages must include one-click unsubscribe. For subscribed messages, messages must contain List-Unsubscribe message headers and a clearly visible unsubscribe link in the message body that can be initiated with a single click (one-click unsubscribe). Unsubscribe actions must be taken for a requesting user within two days.

I won't be able to test for a few hours, but I think my issue is solved.
My domain is registered through GoDaddy. They offer Outlook/MS365 and, for their subscribers, the issue is resolved. I buy MS365 separately from MS, so my DNS servers are handled by the folks who host my website. They are removing their default SPF record, leaving the one added by GoDaddy. This SHOULD solve the problem.

Thank all of you who speak this language and helped me navigate to a solution. This is WAY above my head, but your trail of breadcrumbs led me to the source and I'm very grateful.

I am assuming this issue is still not resolved. I am getting the same message when I send emails from my business email to my personal sky email address and also to a friends yahoo email.

I have tried making my business email a 'safe sender' on my sky email but its still not working. However, this mainly seems to be impacting the calendar invitations as some emails are getting through to recipients.

please let me know if there is anything I need to do in the sky email set up to stop this from happening. I wondered if this was a DNS thing.

Here is the message :

Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.

Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.

For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361.

>> I am assuming this issue is still not resolved.

>> I wondered if this was a DNS thing.

It won't be resolved until the user (or their IT personnel) fixes it.

It most likely is. Do you have proper mail related DNS records (SPF, DMARC etc)?

DO NOT, AND I MEAN DO NOT FOLLOW ANYONES SUGGESTIONS OR FIXES ON HERE!!!!!!

Assuming it was a simple fix I have now messed up my DNS records on 365 because of others assuming their fix is going to fix others.

Be mindful and ask your host first if changing anything or it could mess your DNS records if you are not using Microsoft's.

Also make sure you are not using a CDN (Check you web developer/host) again to see if this will also have a knock on effect.

I am happy for those that have fixed it for them but not everything is setup the same. I have learned the hard way. If anything Microsoft should be sharing a page on how to fix this with different setup i.e. Host/ CDN and Microsoft DNS options.

Be extra careful. I did this on a bad day knowing I will be working over the weekend now to resolve an issue Microsoft seem to struggle with, or at least be aware of and resolve as this issue has now been going on since 2013 and has come back early this year again.

Complete mess and can bring a website down to its knees if you have custom DNS setups.