More formal:
* Concerning: "Concerning your recent email..."
* Regarding: "Regarding the project deadline..."
* With respect to: "With respect to your request..."
* With reference to: "With reference to our previous conversation..."
* In relation to: "In relation to the upcoming conference..."
* Pertaining to: "Pertaining to the budget..."
More informal:
* About: "About the meeting..."
* Regarding: "Regarding the party..."
* On: "On the matter of..."
* Concerning: "Concerning the weather..."
* In terms of: "In terms of the next steps..."
More specific:
* Specifically: "Specifically, I'd like to discuss..."
* Particularly: "Particularly, I'm interested in..."
* Specifically: "Specifically, I'm curious about..."
Avoiding the phrase altogether:
* **Instead of saying "In regards to your email...", you could say "Your email..."
* **Instead of saying "In regards to the meeting...", you could say "About the meeting..."
The best choice will depend on the specific context and desired level of formality.