Intermittent email not being received by me, and some of my email is not received by others

For the last couple of days, some emails sent to me were not received. These are not new senders but colleagues I email many times every day, so just some random ones seem not to be delivered. There is no one in my Blocked Senders list, and nothing in the Junk Folder.  Also, Some of my Sent emails are not being received - not sure how many, but I can reproduce this problem to a person in my office every time I attempt to send her an email,  it doesn't show up, although no error or bounce messages is generated.  And, the email shows up in my Sent Folder as if all is well.