I'm pretty sure that memory usage was optimized since the version you use.
I would say the easiest approach is to download the latest trial and check if it solves your problem:
https://www.limilabs.com/mail/download
80MB attachment is quite big for email. Due to how email works - Base64 encoding is used for email attachment encoding - 80MB attachment results in email that is 106MB.
For example, Gmail limits the size of the message users can send to 25MB.
Since March 2017, they allow receiving emails up to 50MB.
Office 365 allows emails up 150MB in size.
Outlook.com allows emails up 40MB in size.