After 10 tries the email is discarded so you probably don't have many emails in the queue. You can remove any jobs from the queue with SQL. Something like this:
from QRTZ_DYNAMIC_TRIGGERS t
INNER JOIN QRTZ_DYNAMIC_JOB_DETAILS d ON t.JOB_NAME = d.JOB_NAME
where JOB_CLASS_NAME LIKE '%MailGateway%'
delete from QRTZ_DYNAMIC_JOB_DETAILS where JOB_CLASS_NAME LIKE '%MailGateway%'