I was advised by a friend to ask you this question:
For almost 2 years I mailed a friend of mine who is a guy, and I knew that he loved, and I don't deny that liked him because he had good morals. But few months ago we decided to stop mailing each other because it is unreligious and we are both trying to abide by Islamic rules. We agreed that we would only Da'wa. But few days ago I sent some good jokes that are not obscene to few of my friends and him also. He sent me a mail telling me that i have to ask you if this is OK. The Joke did not include any hints about love or anything, just innocent Humour. And I know that at the time of the Prophet, sallallaahu alayhi wa sallam, they appreciated good humour but just not at all times. However, this guy said that my jokes reminded him of me, however he would still remember me if I only sent him religious stuff.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

First of all we draw your attention that it is not permissible for a girl or a woman to establish a friendship, or love relationship with a man through chat rooms, or internet or any other means, as this leads the woman or the man to temptation. You have to know that this is the way of the devil by which he drags the person to fall into sin, fornication, or adultery. Therefore, whoever wants to get married has to tread the proper way and not through the Internet, phone, or other impermissible means. So you have to repent to Allaah for doing so, and immediately end your relationship with this man before it is too late. For more benefit, please refer to Fatwa: 82217.

As regards joking in general, if it does not contain lies or other prohibited things, then there is no harm in it. It is confirmed that the Prophet said: "Woe to him who tells lies to make people laugh thereby, woe to him! Woe to him!" [Abu Daawood]  So it is not permissible to mention a joke which contains lies, people's deficiencies or other prohibited acts. It becomes even more prohibited if it mocks religion, as in this case it would take the person out of the fold of Islam. The humour that is permitted is allowed amongst women or amongst men, or between a woman and her Mahaarim (non-marriageable relatives); but it is forbidden between a man and a marriageable woman. Allaah says (which means): {…then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery, etc.) should be moved with desire, but speak in an honourable manner.}[33:32]. Imaam Al-Qurtubi commented on this verse saying: 'Speaking in an honourable manner is telling the truth that is not denied by Islamic Law or by people regularly.'   

Allaah knows best.