Email Send Sometimes and sometimes Not

Are you getting any exception while sending an email to the mentioned account?
Have you checked Junk / Spam emails of mentioned account?
If email delivery fails then the account from which you are sending an email will get an email containing the details about email delivery failure (like recipient's mailbox is full).
Few organizations block the emails if there are any suspicious words present in email. Considering this possibility please try to send an email with only 2 words.
The account from which you are sending an email might be blocked by the account holder to whom you are sending an email.
If you are sending an email through SMTP on which you have some control then you can find such non-delivered emails in SMTP with exception details.

Please check all the above mentioned points. You will surely find few pointers for resolving email issue.