Opposite of addressee

I wouldn't call it the opposite of "addressee", but I think the word you're looking for is "sender."

teechrI wouldn't call it the opposite of "addressee", but I think the word you're looking for is "sender."

Thank you.

addresser
English
Alternative forms
* addressor
Noun
(en-noun)
A person who gives an address or speech.
A person who addresses (applies an address to an object to be delivered to a particular location).
A machine that addresses.

Because of the above, can I say that an addresser is a sender? Thanks.

Kelly TanBecause of the above, can I say that an addresser is a sender?

No.
How do you intend to use that word?

teechr Kelly TanBecause of the above, can I say that an addresser is a sender?No.How do you intend to use that word?

The addresser of the letter is one of my pen-pal. (Is it ok?)

Kelly TanThe addresser sender of the letter is one of my pen pals.

You could also say,
The person who wrote that letter is one of my pen pals.

teechr Kelly TanThe addresser sender of the letter is one of my pen pals.You could also say,The person who wrote that letter is one of my pen pals.

Thank you, teechr. Can you show me a sentence using addresser?

It's not a commonly used word. Take a look at the example sentences in the link below.

https://en.oxforddictionaries.com/definition/addresser

teechrI wouldn't call it the opposite of "addressee", but I think the word you're looking for is "sender."

Your suggestion is much more common and natural http://dictionary.cambridge.org/dictionary/english/sender