PHP solution for sending excessive email content

If the PHP mail content is too large to be sent, the Ecshop secondary development site is used. When the Mail content is too large, the Mail will fail to be sent (stmp )? Please give me some advice and try to change the execution time to a larger value. The execution time is obviously higher than the sending time of the PHP email, and it fails to be sent if the content of the PHP email is too large.
When Ecshop secondary development site is used, when there are too many emails, the Mail will fail to be sent (stmp )? Expert advice
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Try to increase the execution time
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Change execution time
You obviously sent emails more than the execution time.