Somethings to check:
You are sending to someone who has already unsubscribed. Check their profile, look at their email address and see if they have unsubscribed.
The email address was misspelled, check for commas instead of periods. Check spelling.
You are sending from a yahoo.com, gmail, hotmail or AOL account. Easy fix is to change the email address you are sending from to something at your own domain name, example : [email protected]. For more info, see the footnote at the bottom of this article.
Verify your domain’s DNS configuration in SPARK MINDS to avoid issues related to incorrect domain settings.
Ensure the recipient's mailbox is not full, as a full inbox will block delivery.
Monitor bounce data and logs for detailed troubleshooting, including reviewing recipient details, email subjects, and addresses.
Footnote
Yahoo, Gmail, Hotmail and AOL have changed their email policy years ago, it was intended to crack down on spam, but often times it stops legitimate email too.
We are sending email on your behalf, these email companies do not allow it.
Here is the geeky explanation from yahoo themselves:
https://help.yahoo.com/kb/SLN24016.html
"All DMARC compliant mail receivers (including Yahoo, Hotmail, and Gmail) are now bouncing emails sent as Yahoo email addresses that aren't sent through Yahoo servers."
Unfortunately, there is nothing Spark can do about this (or any other software for that matter), it’s global.
But you can fix this by using your own domain name as your from address. Change it in Spark at:
Left Side Menu Bar > Settings > under General section > General Settings
Under "Business Settings" page, change the "Email Address” field to something on your own domain server.
Avoid repeated testing with the same email address during troubleshooting as this can cause messages to be flagged or rejected.
One good place to get your own domain is at mydomain.com
Another popular option for acquiring custom domains is GoDaddy, known for their domain registration services.