Didn’t hear back via email so I’m going to post this here too:
You should contact your web host if this issue persists.
If it can’t be solved by your host, we could probably get it working in Pro because we can send via a 3rd party using an API, for example: https://sendpress.com/blog/7635/sendpress-mandrill-quick-start-guide/.
you are on a local server (localhost)
recipient’s email address is invalid
there’s a special character in the subject
there’s a special character in the sender/receiver information
the subject line is too long
you already sent too many emails
1. You are on a local server (using WAMP for example)
This kind of web server does not have a mail server so you can’t send emails from your local server.
Solution: send with an external SMTP server instead.
2. Your receiver email address is not valid
Your mail server may refuse to deliver your message if the receiver email address is not a valid one.
If you have “could not instantiate mail function” for only a few of your subscribers, it may just be because their email address is not valid!
3. You included a special character in the subject line
Some mail servers will refuse to deliver your message if it contains special characters in the subject such as a quote or a comma or any other kind of special character (ù,ï,ä)…
Solution: create a new newsletter, specify a standard subject line (“test” for example) and give it a try.
4. You included a special character in the sender/receiver information
The same way, some mail server will refuse to deliver your message if the sender information contain special characters.
Solution: remove special chars from a single subscriber to see if that solves the issue.
5. Your subject line is too long
Some mail server may not authorize you to deliver your message if the subject line is too long.
Solution: shorten your subject line to a single word to make sure it’s not a problem of subject length.
7. You already sent too many emails!
Most of hosting company have daily or even hourly sending limits.
