Line length limit

[t.ae]

According to RFC5322:

Each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters, excluding the CRLF.

If we have multiple to, they are joined to comma separated single line.

Swift-SMTP/Sources/SwiftSMTP/Mail.swift

Line 125 in f9b1d70

 dictionary["TO"] = to.map { $0.mime }.joined(separator: ", ") 

This can easily violate the 998 characters limit.
In my environment, I receive broken mail if there are too many to.
(I think the behavior is up to mail server's implementation)