Hello,
I don't know how your website is structured and how you have configure the subscription ; nor why you need to contact your customers one by one.
What I can tell you is that HikaSerial Subscription have an email notification system for subscription expiration notice.
You also have "mass email" function in Joomla.
You can also find mailing component (like Acymailing) to send a message to every person in a specific list (or user group).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.