I'm curious, what made you go with a multiple lists approach instead of tagging users on a single list?
A lot of popular (but expensive) services like ConvertKit have moved to a tagging approach many many years ago.
It's so much easier to manage your list of emails when it's a single list and then you add tags to specific users like "purchased X". This way the email address only exists in 1 spot and you can segment on the tags.
The Sendy approach (and from the looks of it your app too) becomes very unwieldy with having to manage, parse and merge multiple lists on a regular basis.
+1 for this approach. This was the #1 design decision we made with BigMailer.io back in 2015. It's not trivial to change the model once you go with unique lists approach.
Performance is extra helpful for making real-time segment size calculations. We use elastic search and it's costly.
listmonk supports arbitrary nested tags and attributes in the form of JSON. A subscriber can have properties like {"purchased_x": true, ...}. It's possible to issue complex SQL expressions to filter and send campaigns to subscribers.
The multi-list approach has several other benefits. When manager / sender (and other) permissions get introduced, it will be straight forward to restrict users to managing certain lists. In addition, multiple lists allow subscribers to selectively subscribe / unsubscribe from lists.
Internally, the structure is simple. There is only one subscribers table and subscriber data is not duplicated anywhere. List (foreign key) relationships are in a separate table.
So in theory could you have 1 list and each subscriber has many tags, and then you can segment on those tags?
Also, do you have any public success stories beyond your own Zerodha campaigns? Have you compared delivery / bounce / etc. rates vs Sendy and other tools using the same email providers? Also how fast can you send emails out through SES?
Okay. Again, different target group. Sendy targets people who have no idea what msmtp is. They upload the files to their web host, enter a database password the hoster have them and AWS credentials, and they are done.
Still, the big selling point is: Inexpensive. Easy to install (simple PHP). Looks relatively polished.
All the "competitors" are either much more expensive, or you need to be an enthusiast in the tech stack they have chosen.