I wanna confirm if I can send email to more than one mailing lists at the same time.
I have tried it but it has failed on my side, but when I send to one at a time it does work, so is it generally like that or there is a problem with my settings
If by “mailing list” you mean a category that accepts emails sent to a particular address, the answer is no. Topics exist in exactly one category, so if you email the same message to multiple addresses that would place it in multiple categories it will end up in only one of them.
You will need to send a separate message to each category individually. Another workaround would be to construct the categories (your mailing lists) such that you never need to send the same message to multiple categories/lists.