The question is: Is the from field the same everywhere? And are all your sites on the same host? Your error message looks like mine.
Same host? Why is that relevant. Is your client getting an error message?
Like I said, the issue is probably your From field:
[your-name] <[your-email]>
Either change that to your client's name and email address, or swap WP-Mail-SMTP for Postman SMTP.