There is no objection to this in shari'ah (Islamic law). Rather it is considered from good morals and kindness. It is permissible to congratulate non-Muslims on their religious occasions, using words that do not contradict the Islamic creed. Maintaining ties, giving gifts, visiting and congratulating non-Muslims are all from among kind treatment. Allah the Almighty commands us to speak kind words to everyone alike. He says,
Speak fair to people. [Al-Baqarah, 83).
Allah also commands us to always be kind to others. He says,
Allah commands justice and the doing of good. [An-Nahl, 90)
Allah does not forbid us from maintaining good ties with non-Muslims, exchanging gifts or from other acts of kind treatment. Allah the Almighty says,
Allah forbids you not, with regard to those who fight you not for (your) faith nor derive you out of your homes from dealing kindly and justly with them: for Allah loves those who are just. [Al-Mumtahana, 8).
The Prophet (peace and blessings be upon him and his family) accepted the gifts of non-Muslims
It is established through authentic mutawatir hadiths that the Prophet (pbuh) accepted the gifts of non-Muslims. He accepted the gift of Al-Muqawqis, the great ruler of Coptic Egypt. Ali Ibn Abu Talib (may Allah be pleased with him) reported that Chosroes, Caesar, and other kings sent gifts to the Prophet (pbuh) and he accepted them. [Recorded by Ahmed in his Musnad and At-Tirmidhi in his Sunan).
Scholars of Islam understood from these hadiths the permissibility or recommendation of accepting gifts from non-Muslims because it is an act of benevolence. It is, moreover, a sunnah of the Prophet (pbuh).
The Prophet (pbuh) offered gifts to non-Muslims
Commenting on the gifts the Prophet (pbuh) offered non-Muslims, Al-Sarkhasi said, "Offering gifts to others is from among good morals as per the words of the Prophet (pbuh), “I was sent to establish good manners.” Therefore, scholars understood that exchanging gifts is recommended among Muslims and non-Muslims alike. [Sharh Al-Siyar Al-Kabir, vol. 1, p. 96).
In Al-Fatawa Al-Hindiya, it is mentioned that Mohammed Ibn Al-Hassan (may Allah have mercy on him) said, "There is no harm in visiting and hosting Ahl Al-Dhimma [non-Muslims coexisting peacefully with Muslims] even if they are merely acquaintances. Likewise, there is no harm in a Muslim visiting a non-Muslim whether or not they are close or coexist in peace." [Al-Fatawa Al-Hindiya, vol. 5, p. 347).
In Fath Al-'Ali Al-Malik (vol. 2, p. 349), Sheikh 'Ilish was asked whether congratulating non-Muslims is considered apostasy. He replied, "Congratulating a non-Muslim by wishing him a long life is not considered apostasy since it does not imply venerating or admitting disbelief."
Through the above mentioned noble verses, hadiths and scholarly opinions, it is evident that there is no doubt that maintaining ties with non-Muslims by exchanging visits, offering condolences and good wishes, exchanging gifts and the like are from among kind treatment. This is considered one of the ways towards calling to the religion of Allah through noble manners. Allah the Almighty knows best.