eM Client arbitrarily sends mail to junk

eM Client has been sending selected email to Junk - mail from senders that I have received in my in-box in the past. I have manually restored messages to my in-box only to discover that the program continues to send mail from those addresses to Junk.  I have missed critical messages as a result.  How can I fix this?

Have you possibly marked domains as source of junk? There is normally your provider which sorts certain emails into junk already.

Then you have the possibility to simply move a mail to junk which again makes your provider typically adjusting your junk filter.

Also you have the opportunity to move emails to junk and blacklist the email in eM Client. That causes all future emails in the inbox to be moved by eM Client into the junk mail.

And at last you have the option to move an email to junk and blacklist the domain. Lets say you get emails from advertising(at)domain.com and blacklist the domain then the important mail with your annual invoice from sales(at)domain.com ends up in the junk filter as well.

It can be tricky sometimes to find out why an email had been false positive junk and who did it.

Have a look in [tools] [rules] [black list] if the email address in question is listed there. If not then you may need to enter the email providers web interface and have a look at their settings and if you find valuable mail in the spam folder move it via web interface.

Dirk above mentions lots of very fair points, the first thing I’d check is your server settings - it is possible that your server filters out the messages into Junk first, and eM Client can’t overpower the server rules, so even if you restore those mails in eM Client, server might be still discarding that sender.
If these emails are not in the junk folder on the server/webmail, check the Tools > Rules for Blacklist and see if the important addresses aren’t in it.