What is the probation period, is it six months or one year?
Is the confirmation clause is also mentioned in the employment offer letter?
What about the termination clause or resignation condition, whether you can pay for the unserved notice period?
How did you submit the resignation letter, through email or through written communication and submitted the same in person with acknowledgment of sent by registered post?
The email confirmation by the organisation on post date is not valid and this has been purportedly done only to harass you.
You can claim that you have not received any communication regarding your confirmation till the date you submitted the resignation letter.
Therefore you can argue that you were on probation only till the date you have sent your resignation and demand relieving letter as well as the experience letter by sending a legal notice to previous employer sent by registered post through your lawyer with a copy endorsed to your new employer.
In the meantime you can join the new employment based on this communication with a request stating that you will submit all the relevant documents shortly after receiving them from the previous employer.