It's not possible to have multiple email address in <to> element of SMTPAppender because only the first one is used (https://github.com/ceki/logback/blob/master/logback-core/src/main/java/ch/qos/logback/core/net/SMTPAppenderBase.java#L270)
When using a property, multiple <to> element is not really easy (if even possible)...

Example:

<appender name="email" class="ch.qos.logback.classic.net.SMTPAppender">
...
<to>${mail.log.to}</to>
...
</appender>

With properties :

mail.log.to = admin@domain.com,dev@domain.com,op@domain.com

=> Only admin@domain.com get the email !