Won't send to some e-mails

I have a number of different e-mail address that are me (not unusual). eM client shows X marks alongside them in the dropdown list that appears when I start to enter the address They never seem to get sent out. I have reduced the program to one account - still doesn’t work. Why?

the dropdown list that appears when you start to enter an address is a recipients history.
This utility can store the addresses that you type into the recipient fields of your emails and eM Client can further use these stored addresses to suggest and auto-finish the recipient fields in the future, regardless whether the recipients are in your contact list or not (the contacts with the X mark).

You can click on “Clear recipient history…” in Settings - Send