Personal S/MIME certificates have a defined validity timeframe.
Therefore you have from time to time to switch to a new valid certificate.
In our case that equals to every 3 years.

In SOGo I only can store one S/MIME certificate at a time.
Therefore when switching certificate, I have to first remove the still valid one from the settings.
Then upload the new one.
But this makes all encrypted emails send to me till this point in time unreadable.
Also it will take some time, till all people know of my new certificate.
Because of that it is possible, that some will still use the old still valid one for sending me encrypted emails.
Those I also will not be able to read.

The only possible solution to that is to provide more than one S/MIME certificates simultanious in SOGo.
The minimum requirement are 2.
More seem better to me, because regulations here demand providing some emails for 10 years, which translates in our case to at least 4 S/MIME certificates needed.