emails are bouncing back when sending to yahoo

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.

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.

Multiple SPF records could be your problem. Delete one of them (GoDaddy knows how to do it). That should take care of it.

GoDaddy is my domain provider. I've been reading on their site on how to make the changes. Thank you!

To be clear, Microsoft is my email provider through Office 365, but my domain registrar is Google (which sold that business to Squarespace, and it will be there soon). I had to make DNS changes using Google DNS, although Microsoft told me what changes were needed.

So you may need to deal with two companies. Most people probably use the same company for both services.