I've been troubleshooting my email setup for almost a week. I've successfully set up sending email via relay (Google blocks port 25) using Mailjet. But since day one I haven't been able to login into rainloop or receive any emails. I've tried every configuration I could find, until I realised the reason nothing works is that dovecot can't access the mysql database because of a driver issue:
auth: Fatal: sql: Unknown database driver 'mysql'
I've run dovecot --build-options and mysql, postgre and sqlite are all listed as drivers.
I've tried apt-get install dovecot-mysql but that refuses to budge because of dependencies. This GCP deployment seems to use Mariadb.