Subject: Re: [Original subject]

Dear [Name],

Thank you for your message to [Executive name]. I'm [Your name], [Executive]'s EA.

[Executive] is travelling this week and won't be able to respond personally until [date]. I've flagged your email for them and they'll come back to you directly with a substantive reply.

If this is urgent, please let me know and I'll see what I can do in the meantime.

Best regards,
[Your name]
EA to [Executive name]