Sorry.

Yes, that’s it.

You can also try:

• So sorry.
   
• Really sorry.
   
• I’m sorry I upset you.

Or something along those lines. But definitely not:

• Apologies for any inconvenience this may have caused (a.k.a. The British Rail train announcement)
• We regret this incident (We take no blame so you can’t sue us – ha!)
• Sorry if you took offence (Sorry-not-sorry)
• Sorry you feel that way (You’re just too emotional)
• Sorry but…(Here’s why I’m right and you’re wrong, sucker!)

But even better, pick up the phone. 

Studies on how empathy is transmitted across various media show that voice is most influential.  Even a voice note or voicemail can sound more heartfelt than an email.

So if you aren’t really sorry, don’t say it.

And if you are, say it like you mean it.