The document Configuring an SMTP mail server to send notifications has a heading "Move the JavaMail Classes" under the section Configuring a JNDI location. The section mentions to move the "javax.mail-x.y.z.jar" files out from /lib directory if there's an external application servers.

These javax-mail.* files are not present in Jira's WEB-INF/lib directory any more and they're now replaced with plugin files 
& "jakarta.mail-1.6.5-atlassian-2.jar" and in later versions of Jira, one more jar named "jakarta.mail-api-1.6.5-atlassian-2.jar". These files need to be available directly under the /lib directory of Tomcat application server (for Jira built-in) or any other application server if it's external. Please amend this section in the document.