I hope all is well.

I hope you had a good weekend.

I'm glad we had a chance to chat at the convention.

I am writing to ask/enquire/let you know/confirm/check/invite you to/to update you on/ask for a favor...

Sorry it took me so long to get back to you.

The parts in bold/in red/in blue are my comments/are the changes we made. Please find my two main questions below.
I didn't/don't fully understand [something]. Could you please explain that again?
Please keep me informed/posted/updated/in the loop.
Let me know if this is OK with you.
I'm sorry but it's out of my hands
Any feedback you can give me on this would be greatly/highly/much appreciated.
... please do not hesitate to contact me.
Sorry I couldn't be of more help.