Some message transfers, notably some sendmail implementations, cannot process long header lines properly. This often leads not just to damaged headers but to erroneous message rejection. Although this is a gross violation of standards, it is nevertheless a common problem.
IMTA provides per-channel facilities to split (break) long header lines into multiple, independent header lines. The maxheaderaddrs keyword controls how many addresses can appear on a single line. The maxheaderchars keyword controls how many characters can appear on a single line. Both keywords require a single integer parameter that specifies the associated limit. By default, no limit is imposed on the length of a header line nor on the number of addresses that can appear.