Soondak:, I would like for you to consider another perspective, considering your husband's counsel to stop.
I have a friend who is in his 70s now, who at one time was involved in ministry and oversaw a large work that he primarily founded. This was a dynamic church as it grew, and opened the door for ministry on a National level. He's a good guy.
During this time, his wife became involved in prison ministry, and there met a charismatic young man, and through the bars, so to speak, became obsessed with this young man's life. Little did she know, that she was being used. He was a "confidence" man. A term where we derive "con" man from. A person who has learned to exploit the trust of others, often by playing to their sentiments, and desire for intimacy, and in this case, to be useful to God Himself.
She became so bound by his spell, and the apparent Divine appointment of their relationship, that she divorced her husband, married him and then supported him until the end of his sentence. When he was released from prison, he took her for almost everything she owned, and then dumped her, onward to better hunting grounds.
She returned to her husband ashamed and devastated. They never remarried. He left the ministry.
It could be that his testimony of automatic conversion upon reading your letter was valid. It could be, also that he understands your sentiments, and is seducing you into his web of control. Often, very powerful unclean spirits, and seducing demons accompany these con men, as has been widely documented by recent so called revivals or outpourings. I cannot say.
If he has been truly converted though, he surely would want to correspond with your husband, wouldn't he? He would desire to honor your marriage, wouldn't he? So, let each and every letter, or type of communication from now on, be addressed to your spouse, first. Ask your husband to write him, explaining it is inappropriate for you to do so, and to communicate with him, from now on. This is also the attitude a Godly woman would desire , also.
There are usually very devout and real Christian groups in prisons. ask your husband to take the time and counsel with the prison chaplain, and to refer you both to the leaders of these genuine fellowships in that same prison. They will know, as will the Warden, if this fellow is sincere. Many, many are not, and prey on any that will sympathize with their plight. It is common, and a way of life in Prison.
At any rate, you are a married woman, and have no business nurturing a single man in any circumstance, be it near, or far. It can be dangerous. Your husband will appreciate your submission unto him, and the laying this down.
If this prisoner was converted truely, he will rejoice with your husband too, if your husband feels to minister to him further. You must drop this altogether, in deference to your godly mate.