Comment 3 for bug 1623995

You do not say anything in your bug description that's related to "Mail certificate" as mentioned in the bug title.

Anyway - it's better to open a separate bug report for each issue.

It seems that your gnome-keyring is either not installed or not configured correctly. There is a way to check from the command line that gnome-keyring is working:
$ secret-tool lookup user "org.pantheon.mail imap_username:<email address hidden>"

<email address hidden> should be replaced with the actual e-mail address you use in Mail of course.