[FireBeyond]

I use it with Spark on macOS and iOS (Spark Classic) and it just works. Archiving works fine, marking as spam actually moves and 'activates' Fastmail's training. There's email masks, aliases, the UI is always fast and responsive in the web view when I go there.

[0xhald]

Have you had any issues with Spark or are there any downsides using it? I haven’t used Spark since I started using Fastmail and have been thinking about it

[FireBeyond]

None that I've found. I think just make sure when you configure the account in Spark that the folders are set correctly (i.e. archive and spam folders are mapped to their folders in Fastmail) so you get the correct functionality from them.