In a production environment, large institutional customers expect to be able to inject DATA with very long lines, even megabytes. They don't accept the requirement to comply with RFC5321 4.5.3.1.6, arguing that "other providers accept long lines without issue".

This creates a problem with use of aiosmtpd, because one needs to either instantiate SMTP with a massive maximum line length (leading to a huge preallocated buffer size), or face the issue of asyncio.LimitOverrunError, leading to failure to accept customer mails.

The obvious problem with allowing several megabytes for maximum line length is when an SMTP server has hundreds or even thousands of simultaneous inbound connections, the memory overhead of these buffers will add up to gigabytes.

I'm using a forked aiosmtpd which allows arbitrarily large input lines, by catching asyncio.LimitOverrunError and re-reading chunk by chunk, then when the next .readuntil() call completes without exception, assembles the chunks together.

My question is - would you be willing to consider a pull request if I commit these enhancements into my fork? I can assure full backward compatibility. I would propose adding an extra constructor keyword such as 'long_lines=False', and if set to True instead, the long line chunk collection would take effect.