Clarification of the matters that are not permissible in congratulations.

The Shaykh : Is people congratulating one another a matter of worship or a matter of custom? It is a matter of custom, even though it is reported from some of the Companions that they used to congratulate one another on this occasion. However, it is a matter of custom, but some people do in this custom what is not permissible in the Shari'ah. For example, a man congratulates his cousin’s daughter while she is uncovering her face. What do you say about this? This is forbidden and not permissible. It is not permissible for a man to congratulate his cousin’s daughter while she is uncovering her face, because she is a non-mahram to him; she is not one of his mahrams. Some people also congratulate any female relative, even if she is not their cousin, and this is also forbidden if she is not one of their mahrams. It is forbidden for him to do so while she is uncovering her face.
Now, some people also congratulate female relatives who have reached the age of maturity but are not their mahrams, and they shake their hands. Is this lawful or forbidden? It is forbidden. It is not permissible for a man to shake hands with a woman who is not one of his mahrams. But if he says, “I shake her hand from behind a barrier,” is this forbidden or not? It is also forbidden, because a person may be led astray by Satan. If he shakes her hand, he may press it, and what happens happens. Therefore, it is not permissible for a person to shake hands with a woman who is not one of his mahrams, neither from behind a barrier nor directly. Now.