I hope someone can help, and that I've chosen the relevant Stack Exchange site for my question.
I'm having a strange issue with emails not being received by some people. I have a website that sends out emails, and I've never had a problem with it. I'm BCC'd in on every email that gets sent to I know they are being sent and I'm receiving them myself, however some people have reported that they are not receiving the emails for some reason. These are users that have always received them without issue in the past, and I know they've been sent because I have a copy of the email. They insist that no filtering happens and that they are not going to their Junk folders. Strangely, I can email the users from my email client without issue but if I try and forward the missing mail it doesn't seem to get through to them either. I've checked against blacklists and not had any negatives returned. It is happening for one user both on his office email account and on his personal email account.
My question is this: what else can I do to diagnose this? I am at a complete loss, I'm insisting it's not something at my end as I'm getting the emails through but I'm starting to wonder if that's true as three people have reported the issue today, and they've never had an issue before. This isn't affecting everyone, but three of seven emails so far today have not been received by the users. The website has been running successfully for well over a year now.
Any and all advice would be greatly appreciated!