Receiving emails not addressed to me (was not BCC'd)

Hi all,

I'm the admin for our Microsoft 365 environment. Twice in the past month I have received an email from 2 separate colleagues when the email was not addressed to me. I have spoken with both of these colleagues and neither of them had BCC'd me into their emails. I also have no mail forwarding rules or mail rules set up in our environment that would cause this to happen.

Email 1:

colleagueA(at)mycompany.com sent an email to hisbrother(at)gmail.com - I also received this email but my colleague assured me he did not BCC me

Email 2:

colleagueB(at)mycompany.com sent an email to collegueC(at)mycompany.com and externalclient(at)client.com - I also received this email and have just spoken to my colleague on the phone and they have assured me that they did not BCC me into this email.

Has anyone ever heard of this happening before? I've tried searching for answers but everyone goes for the easy "you must've been BCC'd" answer. This is 100% not the case here. I'm also concerned that if this is happening to me, then are other people in my organisation receiving emails not addressed to them? I've yet to have anyone report this to me, but it is extremely strange behaviour. Any ideas on what to look into would be greatly appreciated.