File reading offset in case of application restart

Question is more on designing part than coding.

I have one huge File containing Mobile numbers, So what we do is read 50 numbers in one go and update the offset in database table startOffset=0 and endOffset=50. Now, start sending SMS to numbers one by one and once done, another 50 numbers are picked from file and update Offset as startOffset=51 and endOffset=100, now start sending SMS, say I have sent sms till 66 offset, PROBLEM comes when power goes off or application restart suddenly due to many reasons.

Now when application restarts, I will not come to know on what offset I have to start again (ie 67 in our case.). I will come to know that I lost in 51 to 100 chunk, So what I can do is start sending sms again from 51 but this is wrong as sms will be sent twice to number which we already sent.

Also, I can do like after sending each sms, update the currentPointer to DB, but this will make the system slow.

Can someone please suggest the best way to do this.