I was hitting this error with phpMailer + Amazon SES. The phpMailer error is not very descriptive:
2: message: SERVER -> CLIENT: 554 Transaction failed: Expected ';', got "\"
1: message:
2: message: SMTP Error: data not accepted.
For me the issue was simply that I had the following as content type:
$phpmailer->ContentType = 'text/html; charset=utf-8\r\n';
But that it shouldn't have the linebreak in it:
$phpmailer->ContentType = 'text/html; charset=utf-8';
... I suspect this was legacy code from our older version. So basically, triple check every $phpmailer setting you're adding - the smallest detail counts.