I was lucky to find an edition of this game that gave me at a discount price a few nice features including an extra character slot. Now that the 1 month description has ended I still got that slot and a playable character.

However, I like to delete the character and start another one. But I am afraid that I will lose the slot if I try. Is this what will happen or not?

if u delete the character u will lose everything that character had (including himselft )
but the slot still remain, so u will able to create another one

Mind you, I had 3 slots as Gold but only 2 normally as free player.

Keeping that third 'mixed' character slot depends on when you were 'Gold'... Up through the end of last year, you would receive 3 mixed character slots, with the third slot unlocked at Level 6. With the introduction of F2P, this changed to a dedicated Fed and a dedicated KDF slot, with the 3rd mixed slot added with a Gold subscription. If you became a 'Gold' account during F2P, you can lose the 'mixed' slot should it ever become vacated.

Also be aware that your characters EC Bank account cap will drop from 1 Billion ECs to 10 Million ECs when you transition from 'Gold' to 'Silver'. If your characters balance is above 10 Million when you transition to 'Silver', you keep your current balance, but any ECs you would have earned will be lost until your balance drops below 10 Million. If your balance is below 10 Million ECs, and you should earn something that will take your account above 10 Million ECs, then you only get up to the level cap, and any amount over 10 Million will be lost...

It is as I expected then. A friend lost his Billion EC privilege. I never obtained that much, thought I won't be deleting that character! Thanks.

What about the in-game mail ?
I saw in the feature matrix that F2P (silver) has limited mail where gold has unlimited. Is that the number of stored messages or attachments ?

According to the F2P Feature Matrix, the limitations are who you can send Email to, (No doubt to limit in-game Email SPAM from Gold Farmers) and having (or had) a subscription, or making a purchase from the Z-Store removes these restrictions...
http://sto.perfectworld.com/about/fr...eatures-matrix