Restoring Default Mail Server
JIRA is unable to create content (issues, etc.) by e-mail.
Verify the SMTP server configuration.
JIRA's default mail server configuration has been deleted.
If the SMTP settings have been deleted, JIRA is unable to rebuild them without a restart.
Raise a ticket at https://support.atlassian.com and ask for an instance restart.
The Cloud management environment will check the needed configuration and will re-create the missing ones.