It could happen sometimes that your IP is blacklisted by some email provider, or anti-spam services. If you receive an alert from the diagnosis tool, click on details to find the unblocking forms.
To check your Email deliverability, YunoHost provide some tests avilables in the Diagnosis tool. This tools evalutes a lot of configuration and ip reputation points. It gives you also an indicator if some mails are blocked inside the mail queue (waiting to be sent). all the points evaluated by the well known mail-tester.com except for mail content (usefull if you prepare a newsletter).
However, if you have a doubt on the internal diagnosis results, you could check on external tools:
This command can help you to summarize which emails has been refused by other SMTP server and why.
cat /var/log/mail.log | grep "deferred" | sed -E "s/(:[0-9][0-9]).+</\1\t/g" | sed -E "s/>.+dsn/\terror/g" | sed -E "s/, status=deferred \(/ /g" | sed -E "s/\)$//g"
See the list of SMTP return code from wikipedia.
YunoHost is only able to test generic blacklist using the DNS RBL mechanism. However, Gmail, Microsoft, Yahoo or Free maintains their own blacklisting mechanism, so in some situation you may need to contact their teams through dedicated forms or use dedicated tools.
You can find a tool to test your IP, advices, explanation of error code and a way to contact Free on this page
If you use your own domains and think that some mails are sent by unauthorized servers (so without SPF/DKIM), you get report about this mail with.
_dmarc.DOMAIN 3600 IN TXT "v=DMARC1; p=none; fo=1; rua=mailto:firstname.lastname@example.org!10m"
Found errors? Think you can improve this documentation? Simply click the Edit link at the top of the page, and then the icon on Github to suggest changes.