The real problem isn't the protocol, but the cowboy approach to interacting with it. It's not hard to "accept text and encode it properly per the SMTP protocol", you just need to realize you need to do it in the first place.
There is a multitude of classes of errors and security vulnerabilities, including "SQL injection", XSS, and similar, that are all caused by the same mistake that this case of missing period was[0]: gluing strings together. For example, with SQL queries, the operation of binding values to a query template should happen in "SQL space", not in untyped string space. "SELECT * FROM foo WHERE foo.bar = " + $userData; is doing the dumb thing and writing directly to SQL's serialized format. In correct code (and correct thinking), "SELECT * FROM..." bit is not a string, it just looks like one. Same with HTML templating[1] - work with the document tree instead of its string representation, and you'll avoid dumb vulnerabilities.
So, if you want to avoid missing dots in your e-mails, don't inject unstructured text into the middle of SMTP pipeline. Respect the abstraction level at which you work.
See also: langsec.
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[0] - And therefore should be considered as a single class of errors, IMO.
[1] - Templating systems themselves are thus a mistake belonging to this class, too - they're all about gluing string representations together, where the correct way is to work at the level of language/data structures represented by the text.