Forms of Address
Generally speaking, in the UK when a married couple obtains a divorce, the man's style of address does not change. He continues to be styled (on an envelope, for example), as either John Smith Esq, Mr John Smith, or John Smith - or whatever style he went by during his marriage.
The style of address for a woman whose marriage has been dissolved by divorce is considerably more open to change. By convention a divorcee would adopt the style of, for example, Mrs Caroline Smith (ie retaining her title of 'Mrs' and her married surname 'Smith', but using her own forename, ie no longer 'Mrs John Smith').
This rule is certainly commonly followed by ladies senior in years, but it is by no means the rule for younger women. Many women of today, especially those who combine a working life with a married life, choose not to use their husband's surname at all, right from the outset, retaining their maiden name throughout the marriage.
In this circumstance, obviously, the dissolution of a marriage will have no effect on the style of address for the female partner; the only question is whether she prefers to be Mrs, Miss or Ms Janie Jones, for example, or just Janie Jones with no title.
Some women who obtain a divorce (and who have during their married lives followed convention and assumed their husband's surname), may prefer to revert to their maiden name. This happens most often where there are no children from the marriage and the divorce has been particularly acrimonious.
If there are children, however, many women will retain their married surnames just in order to maintain the status quo and to avoid a situation where the mother is using a different surname to that of her children.
To summarise, therefore, it is recommended that, when writing to a divorcee, you should follow the conventional option, and call her'Mrs Caroline Smith' (ie use her forename and her married surname). It is strongly advised, however, that if there is any doubt as to the woman's preferred form of address, you should ask the woman in question precisely how she wishes to be addressed, or ask a reliable friend.