Find email address between specific substring and semicolon

I'd like to extract the email address frim this string:

ezeze smtp.mailfrom=mymail@gmail.com; rerer

How should I write my regex in preg_match to find the email?

You can try this regex pattern:

(?<=mailfrom=)[^;]+(?=;)

(?<=mailfrom=) // assert 'mailfrom=' on the left
[^;]+          // 1 or more characters not ';'
(?=;)          // assert ';' on the right

Demo