However, having investigated Curry's claim that Mbonambi called him "a white (followed by an expletive)", World Rugby on Thursday declared the matter "closed".
"Having considered all the available evidence, including match footage, audio and evidence from both teams, the governing body has determined that there is insufficient evidence at this time to proceed with charges," rugby's global governing body said in a statement.
"Therefore, the matter is deemed closed unless additional evidence comes to light."
Their decision sparked a furious reaction from the RFU, who claimed Mbonambi had aimed the same slur at back row forward Curry in a Test match between the countries last November.
"The RFU are deeply disappointed by the decision taken by World Rugby," it said in a statement.
"The decision not to put the evidence before an Independent Disciplinary Panel has denied the disciplinary process the opportunity to hear Tom Curry's voice and to independently assess his account of these serious events, together with the other available evidence."
Springbok head coach Jacques Nienaber said the issue had not affected him and the squad.
"We're fortunate in terms of the support of a legal team and everybody who worked behind the scenes," he said at the announcement of the team on Thursday.
"We were shielded from it, we were in our bubble. For us we focused on rugby and that was taken care of in the background."
The decision was not the only matter the RFU was furious about.
Curry, who four years ago was in the England team that lost to a South Africa side including Mbonambi in the World Cup final in Japan, has been bombarded with "disgusting" abuse on social media.
"In their continued full support of Tom, the RFU together with the England Squad, condemn the disgusting abuse he and his family has received on social media as a result of his having had the courage to put unacceptable behaviour that has no place in society or on the rugby field, in the public eye," the RFU said.
"Abuse of any kind is not acceptable and goes against the core values of rugby.
"It is important that it is safe and acceptable for everyone involved in rugby union to raise concerns, and the RFU continue to encourage everyone to report any unacceptable behaviour in the game."
Curry, who will play in Friday's third place play-off against Argentina, received support from Springboks captain Siya Kolisi.
"I have spoken to him, I sent him a message," Kolisi said at Thursday's press conference.
"He is someone I respect. We can take it as players. When it comes to you it's fine, but when families are involved it's different.
"I have let him know we are supporting him, we are thinking of him."