What is the ruling on continuing to write in a forum where an illicit relationship once developed, given that this relationship has now ended and the writing is solely for da'wah purposes?

The temptation of women is among the greatest and most dangerous temptations. The Prophet, peace and blessings be upon him, said: "I have not left behind me any temptation more harmful to men than women," and "Beware of the world and beware of women." No one should feel safe from this temptation, for a glance or a word can tempt a righteous man. For this reason, Islamic law prohibits private seclusion with a foreign woman, looking at her, and her speaking in a soft, alluring manner. For a complete repentance, it is preferable for each of them to leave the shared forum and seek another forum, as remaining in the same forum after falling into error might be exploited by Satan. This is more cautious, even if innocent writing, within the bounds of Islamic law, is not forbidden.