Using WordPress 5.4 and CiviCRM Version 5.18.4. CiviMail has been working fine, mostly, using Mail(). This problem is seen when sending mail to an individual from their contact page, or when sending a test mail of what will hopefully be a bulk mailing.
One particular email message fails to send. It is fairly long, and the content is fairly complex, with lots of links and some standard html formatting.
When sending, I get either "An error occurred when CiviCRM attempted to send an email (via SMTP). ... The mail library returned the following error message: mail() returned failure" OR (if a "test" mailing) "Could not identify any recipients. Perhaps the group is empty?"
I searched the html source for anything weird, but found nothing. I can send the entire content in small pieces, but when I put them all together, it fails. I can send the entire content as plain text. It's possible that there is some other reason for the odd pattern of failures, but "too much content" or "too many links" seems like a possible one. But I don't see anything about it in documentation.
Has anyone seen something like this? Any other troubleshooting steps you can suggest to pinpoint the problem? Thanks!