Currently Wekan SMTP server settings are set with command line by server admin. This is the first time someone had asked to have multiple email servers. I'm thinking how useful this would be? Also, if any Wekan instance user that creates new board and that way becomes Board Admin, would be able to add any random SMTP server, in that case there would be a) possibility to send spam b) possibilitity to end up to some spam blacklist with misconfigured settings
If you would like to have this feature, you can prepay 1000 euro at https://wekan.team/commercial-support/ so I will develop this feature and add it to Wekan. When feature has been developed, it will come in automatic update for Wekan Snap version.