The WebUI doesn't allow the use of spaces, comma, and java.text.MessageFormat tokens in properties for CustomHandlers or CustomFormatters.  The result is the following:

Invalid format. Add new properties as key=value pairs, the special character allowed in the value are: - . : @ ; = ? ! # $ % & [ ] ( ) * _ 

The MailHandler uses the properties defined by the SMTP transport implementation such as AngusMail.  AngusMail use space to generate a list of items such as [mail.smtp.auth.mechanisms=LOGIN PLAIN DIGEST-MD5 NTLM|https://eclipse-ee4j.github.io/angus-mail/docs/api/org.eclipse.angus.mail/org/eclipse/angus/mail/smtp/package-summary.html]

In Angus Mail, comma is used in the case of `mail.smtp.dsn.notify=SUCCESS,FAILURE,DELAY`

These are properties that the MailHandler supports by passing along to the SMTP transport implementation but WildFly will not allow these to be set in the Web UI.

AngusMail ships with a CollectorFormatter which is used to build subject lines for emails.  This formatter leans on the java.text.MessageFormat.  Here are some example patterns 

• {0}{1}{2}{4,choice,-1#|0#|0<... {4,number,integer} more}\n

   

• These {3} messages occurred between\n{7,date,EEE, MMM dd HH:mm:ss:S ZZZ yyyy} and {8,time,EEE, MMM dd HH:mm:ss:S ZZZ yyyy}\n

• These {3} messages occurred between {9,choice,86400000#{7,date} {7,time} and {8,time}|86400000<{7,date} and {8,date}}\n

In this case "{}|<\" are not allowed for use.  Those are examples the full list would include all characters from https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/text/ChoiceFormat.html, https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/text/DecimalFormat.html (table of specials), and java.text.SimpleDateFormat (+-) should be supported to allow setting properties that are java.text.MessageFormat patterns.