It would be very helpful to see your code, or a shortened example that repoduces the problem. See also more hints on how to help us help you here.

Having said that, if you're opening sendmail and piping your mail there, that method is somewhat outdated and there are several modules to help you send email. See e.g. How do I send email?

In reply to Re: Sendmail only sending sometimes by Anonymous Monk
in thread Sendmail only sending sometimes by Sirwunton

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