You should copy the whole error message. If it is
"Maximum number of connections from user+IP exceeded" you should increase the 'mail_max_userip_connections' parameter.
At the other hand, if you have 90 users and some of then use laptop + smartphone it's natural to have these warnings if you have configured some related parameter to 100.
To customize these parameters, or others, you have to take account that Zentyal generates the configuration files of the services which manages through templates "stubs" and a redis database. So, the changes which you'll make in the configuration files of the services will be removed when restarting the services.
Zentyal is designed to use first the customized templates which have to be located in "/etc/zentyal/stubs/...", afterward it search the pre-configured templates in "/usr/share/zentyal/stubs/...". Read the docs in order to understand the way of doing:
https://doc.zentyal.org/en/appendix-c.html#stubsCheers!