Usage of supposed in 'I was supposed to send a mail to a friend of mine whose...'

what is the use of "supposed to’’ in the sentence " I was supposed to send a mail
to a friend of mine whose name is Sarah? In which situation we can use it. please define it full well. :?:

thanks a lot sir.

be supposed to do/be something

a) used to say what someone should or should not do, especially because of rules or what someone in authority has said:
We’re supposed to check out of the hotel by 11 o’clock.
I’m not supposed to tell anyone.
What time are you supposed to be there?

b) used to say what was or is expected or intended to happen, especially when it did not happen:
No one was supposed to know about it.
The meeting was supposed to take place on Tuesday, but we’ve had to postpone it.
The new laws are supposed to prevent crime.

c) used to say that something is believed to be true by many people, although it might not be true or you might disagree:
The castle is supposed to be haunted.
‘Dirty Harry’ is supposed to be one of Eastwood’s best films.
Mrs Carver is supposed to have a lot of money.

In your context “supposed to” means that you were expected/required to send a letter to your friend Sarah.

Hi Milanya,
you know I have read many books on this topic but it was never as clear to me as you made it.
Thanks is a small word I guess.