Emails showing as sent in Apple Mail but are not always received

I have an issue with Apple Mail where not all recipients are receiving emails.

The problem seems to be random in the recipients that are and are not receiving email. So for example, I recently sent an email to 8 people, 5 of them received it, and 3 did not. With the three that did not, even if I send them an email individually, they don't receive it. However, if I login to email on a browser and send via the broweser instead of through the Apple Mail client, they will receive it.

I have a number of accounts set up in Apple Mail, and these recipients don't receive email from me, no matter which account it is sent from.

A number of other colleagues who use Apple Mail are experiencing a similar issue.

Does this make sense? Let me know what further information is needed to try to diagnose the problem.