Mailspring no longer attempts CRAM-MD5 SMTP auth instead of PLAIN or LOGIN when both are supported. CRAM-MD5 and DIGEST-MD5 require you've exchanged a shared secret with the SMTP server which is almost never the case.
Mark as Spam now appears in the right-click dropdown menu for threads.
The IMAP/SMTP port dropdowns no longer "stick" the first time you change them during setup.
Mailspring now lets you know if your SMTP server refused to relay a test message during setup.
Mailspring now correctly supports SMTP accounts that do not require a username or password.
Open and link tracking now work correctly for accounts using Courier IMAP, (where sent messages do not appear until re-selecting the folder via IMAP.)
Mailspring now prints descriptive SMTP error strings instead of SMTP error codes in logs.