Technically there are no limits in Mailster. You can have unlimited subscribers, lists, forms and campaigns. There are some server related limits though.
Mailster has been built with performance in mind. It can handle thousands of contacts. We also have some customers with over a million subscribers in many lists.
In general you can say:
The more data you store the bigger you database will get and the slower is the performance.
If you have more subscribers in more lists with more custom fields Mailster needs more time to process your campaigns. If you have complex conditions and different autoresponder setups Mailster needs more time and resources (CPU, Memory) to process these campaigns.
How many emails can Mailster handle?
There’s no dedicate number as it depends mostly on your server structure. Mailster requires at least PHP 5.3+ and a Memory Limit of 128MB or more.
It’s always better to have a dedicate server or VPS where your WordPress site is installed.
How to improve performance?
Store less subscribers as possible
Remove inactive subscribers or subscribers which have a different status as “subscribed” or “pending”.
Store less custom field data
Remove unused custom field data on the settings page.
Store less meta data of your subscribers.
Disable tracking and IP storage of subscribers.
If possible store your data in custom fields and not in lists.
Reduce autoresponder campaigns
Autoresponder campaigns can take many resources on processing depending on the complexity. Try to use as less conditions as possible.
Remove old campaigns.
All campaigns have additional data stored like opens, clicks etc. If you delete old unused campaigns you can free up some database space.
Improve sending speed.
If you speed up your delivery you can handle more subscribers in general.