Yahoo and AOL have made changes to their authentication policies which may affect the deliverability of your newsletter.
Essentially, the changes cause any emails using a Yahoo.com or Aol.com From address to be rejected if they’re not being sent through their own servers. TinyLetter allows you to use any valid email address as your From address, though your emails are sent using our servers. So if you use a Yahoo.com or Aol.com From address in TinyLetter, your newsletter will bounce for recipients adhering to this DMARC policy.
The best way to prevent this from happening is to change your From address in TinyLetter to an address from your own domain. We recommend using a privately hosted domain, since an ISP can make changes like these at any time.
You can change your address at any time by going to Username > Your Profile, editing the address shown under Email Address, then clicking Save at the bottom of the page. We’ll automatically send a verification email to that address, and clicking the verification link in that email completes the process (this is important, to keep your subscribe form active).
To make sure your newsletters are successfully delivered to your subscribers, we’ll automatically change your From email address if using a Yahoo or AOL email address. You’ll see send.tinyletterapp.com added to the From email address.
For example, if your From email address is email@example.com, TinyLetter will change your address to firstname.lastname@example.org when the newsletter is sent to your subscribers. If someone replies, it will go straight to your Yahoo address without a problem.
You can read more about these changes by reading Yahoo’s help article as well as AOL's blog post.