Cheaper (€30/yr vs ~€50/yr; if you don’t need custom domains, €1/mo)
More aliases (25 on mailbox, 50 on own domain. Proton has 10 TOTAL - why custom domain aliases are counted against Proton ones does not make sense to me.)
Support for any number of custom domains AFAICT (Proton Plus supports only one)
Trial account is not allowed to send emails, so fewer issues with services blacklisting proton.me and protonmail.com for spam (hasn’t happened to me, but I have heard of some cases)
Can use a regular email client (security tradeoff for E2EE messages - but there already were plenty of discussions on whether E2EE has benefits, especially sending mail to other services)
Mailbox.org
Mailbox Standard compared to ProtonMail Plus: